RESTful Java Web Services - Second Edition

4.7 (6 reviews total)
By Jobinesh Purushothaman
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  1. Introducing the REST Architectural Style

About this book

REST (REpresentational State Transfer) is a simple yet powerful software architecture style to create scalable web services and allow them to be simple, lightweight, and fast. The REST API uses HTTP and JSON, so that it can be used with many programming languages such as Ruby, Java, Python, and Scala. Its use in Java seems to be the most popular though, because of the API's reusability.

This book is a guide to developing RESTful web services in Java using the popular RESTful framework APIs available today. You will begin with gaining an in-depth knowledge of the RESTful software architectural style and its relevance in modern applications. Further, you will understand the APIs to parse, generate, transform, and query JSON effectively. Then, you will see how to build a simple RESTful service using the popular JAX-RS 2.0 API along with some real-world examples. This book will introduce you to the Jersey framework API, which is used to simplify your web services.

You will also see how to secure your services with various authentication mechanisms. You will get to grips with various solutions to describe, produce, consume, and visualize RESTful web services. Finally, you will see how to design your web services to equip them for the future technological advances, be it Cloud or mobile computing.

By the end of this book, you will be able to efficiently build robust, scalable, and secure RESTful web services, making use of the JAX-RS and Jersey framework extensions.

Publication date:
September 2015
Publisher
Packt
Pages
354
ISBN
9781784399092

 

Chapter 1. Introducing the REST Architectural Style

In this chapter, we will cover the Representational State Transfer (REST) software architectural style, as described in Roy Fielding's PhD dissertation. You may find a brief discussion on HTTP before getting into the details of REST. Once the base is set, we will be ready for the next step. We will then discuss the set of constraints, the main components, and the abstractions that make a software system RESTful. Here is the list of topics covered in this chapter:

  • The REST architectural style

  • Introducing HTTP

  • The evolution of RESTful web services

  • The core architectural elements of a RESTful system

  • Description and discovery of RESTful web services

  • Java tools and frameworks for building RESTful web services

 

The REST architectural style


REST is not an architecture; rather, it is a set of constraints that creates a software architectural style, which can be used for building distributed applications.

Tip

You can read Architectural Styles and the Design of Network-Based Software Architectures, Roy Fielding, 2000, which talks about the REST architectural style by visiting http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm.

Fielding arrived at REST by evaluating all networking resources and technologies available for creating distributed applications. He observed that without any constraints, one may end up developing applications with no rules or limits that are hard to maintain and extend. After considerable research on building a better architecture for a distributed application, he ended with the following constraints that define a RESTful system:

  • Client-server: This constraint keeps the client and the server loosely coupled. In this case, the client does not need to know the implementation details in the server and the server is not worried about how the data is used by the client. However, a common interface is maintained between the client and the server to ease the communication.

  • Stateless: There should be no need for the service to keep users' sessions. In other words, each request should be independent of the others.

  • Cacheable: This constraint has to support a caching system. The network infrastructure should support a cache at different levels. Caching can avoid repeated round trips between the client and the server for retrieving the same resource.

  • Uniform interface: This constraint indicates a generic interface to manage all interactions between the client and the server in a unified way, which simplifies and decouples the architecture. This constraint indicates that each resource exposed for use by the client must have a unique address and should be accessible through a generic interface. The client can act on the resources by using a generic set of methods.

  • Layered system: The server can have multiple layers for implementation. This layered architecture helps to improve scalability by enabling load balancing. It also improves performance by providing shared caches at different levels. The top layer, being the door to the system, can enforce security policies as well.

  • Code on demand: This constraint is optional. This constraint indicates that the functionality of the client applications can be extended at runtime by allowing a code download from the server and executing the code. Some examples are the applets and the JavaScript code that get transferred and executed on the client side at runtime.

The following diagram illustrates a high-level architectural view of a RESTful system:

The preceding constraints do not dictate what kind of technology to use; they only define how data is transferred between components and the benefits of following guidelines. Therefore, a RESTful system can be implemented in any available networking architecture. More importantly, there is no need for us to invent new technologies or networking protocols. We can very well use the existing networking infrastructures, such as the World Wide Web (WWW), or simply the Web, to create RESTful architectures. Consequently, a RESTful architecture is one that is maintainable, extendable, and distributed.

Before all REST constraints were formalized, we already had a working example of a RESTful system, the web. Now, you may ask why introduce these RESTful requirements to web application development when it is agreed that the Web is already RESTful.

Here is the answer: We need to first qualify what it means for the web to be RESTful. On the one hand, the static web is RESTful because static websites follow Fielding's definition of a RESTful architecture. For instance, the existing web infrastructure provides caching systems, stateless connection, and unique hyperlinks to resources, where resources are all of the documents available on every website and the representations of these documents are already set by files being browser readable (the HTML files, for example). Therefore, the static web is a system built in the REST-like architectural style. In simple words, we can say that REST leverages these amazing features of the web with some constraints.

On the other hand, traditional dynamic web applications have not always been RESTful because they typically break some of the outlined constraints. For instance, most dynamic applications are not stateless because servers track users through the container sessions or client-side cookie schemes. Therefore, we conclude that the dynamic web is not normally built in the REST-like architectural style.

Note

The REST architectural style is not specific to any protocol. However, as HTTP is the primary transfer protocol for the web today, REST over HTTP is the most common implementation. In this book, when we talk about REST, we refer to REST over HTTP unless otherwise stated.

Now, you may be curious to learn more about a RESTful system. The rest of the chapter will definitely help you to know the internals. However, the topics on the RESTful system, which we are going to discuss in the coming sections, may need some basic knowledge of HTTP. So, let's take a crash course on HTTP to learn some basics and then proceed with our discussions thereafter. You can skip the next section if you are already familiar with HTTP.

 

Introducing HTTP


Hypertext Transfer Protocol (HTTP) is the foundation of data communication for WWW. This protocol defines how messages are formatted, transmitted, and processed over the Internet. Let's have a quick recap of HTTP in this section.

HTTP versions

HTTP has been consistently evolving over time. So far, there are three versions. HTTP/0.9 was the first documented version, which was released in the year 1991. This was very primitive and supported only the GET method. Later, HTTP/1.0 was released in the year 1996 with more features and corrections for the shortcomings in the previous release. HTTP/1.0 supported more request methods such as GET, HEAD, and POST. The next release was HTTP/1.1 in the year 1999. This was the revision of HTTP/1.0. This version is in common use today.

HTTP/2 (originally named HTTP 2.0) is the next planned version. It is mainly focused on how the data is framed and transported between the client and the server.

Tip

To learn more about HTTP, you can refer to the Wikipedia resources that you may find at http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol.

Understanding the HTTP request-response model

HTTP works in a request-response manner. Let's take an example to understand this model better.

The following example illustrates the basic request-response model of communication between a web browser and a server over HTTP. The following sequence diagram illustrates the request and response messages sent between the client and the server:

Here is a detailed explanation of the sequence of actions shown in the preceding diagram.

The user enters the following URL in the browser, http://www.example.com/index.html, and then submits the request. The browser establishes a connection with the server and sends a request to the server in the form of a request method, URI, and protocol version, followed by a message containing request modifiers, client information, and possible body content. The sample request looks like the following:

GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/htmlAccept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive

Let's take a minute to understand the structure of the preceding message. The following code is what you see in the first lines of the request in our example:

GET /index.html HTTP/1.1

The general format for the request line is an HTTP command, followed by the resource to retrieve, and the HTTP version supported by the client. The client can be any application that understands HTTP, although this example refers to a web browser as the client. The request line and other header fields must end with a carriage return character followed by a line feed character. In the preceding example, the browser instructs the server to get the index.html file through the HTTP 1.1 protocol.

The rest of the information that you may see in the request message is the HTTP header values for use by the server. The header fields are colon-separated key-value pairs in the plain-text format, terminated by a carriage return followed by a line feed character. The header fields in the request, such as the acceptable content types, languages, and connection type, are the operating parameters for an HTTP transaction. The server can use this information while preparing the response for the request. A blank line is used at the end of the header to indicate the end of the header portion in a request.

The last part of an HTTP request is the HTTP body. Typically, the body is left blank unless the client has some data to submit to the server. In our example, the body part is empty as this is a GET request for retrieving a page from the server.

So far, we have been discussing the HTTP request sent by the client. Now, let's take a look at what happens on the server when the message is received. Once the server receives the HTTP request, it will process the message and return a response to the client. The response is made up of the reply status code from the server, followed by the HTTP header and a response content body:

HTTP/1.1 200 OK
Accept-Ranges: bytes
Cache-Control: max-age=604800
Content-Type: text/html
Date: Wed, 03 Dec 2014 15:05:59 GMT
Content-Length: 1270

<html>
<head>
  <title>An Example Page</title>
</head>
<body>
  Hello World !
</body>
</html>.

The first line in the response is a status line. It contains the HTTP version that the server is using, followed by a numeric status code and its associated textual phrase. The status code indicates one of the following parameters: informational codes, success of the request, client error, server error, or redirection of the request. In our example, the status line is as follows:

HTTP/1.1 200 OK

The next item in the response is the HTTP response header. Similar to the request header, the response header follows the colon-separated name-value pair format terminated by the carriage return and line feed characters. The HTTP response header can contain useful information about the resource being fetched, the server hosting the resource, and some parameters controlling the client behavior while dealing with the resource, such as content type, cache expiry, and refresh rate.

The last part of the response is the response body. Upon the successful processing of the request, the server will add the requested resource in the HTTP response body. It can be HTML, binary data, image, video, text, XML, JSON, and so on. Once the response body has been sent to the requestor, the HTTP server will disconnect if the connection created during the request is not of the keep-alive type (using the Connection: keep-alive header).

Uniform resource identifier

You may see the term uniform resource identifier (URI) used very frequently in the rest of the chapter. A URI is a text that identifies any resource or name on the Internet. One can further classify a URI as a Uniform Resource Locator (URL) if the text used for identifying the resource also holds the means for accessing the resource such as HTTP or FTP. The following is one such example:

https://www.packtpub.com/application-development

In general, all URLs are URIs. To learn more about URIs, visit http://en.wikipedia.org/wiki/Uniform_resource_identifier.

Understanding the HTTP request methods

In the previous section, we discussed about the HTTP GET request method for retrieving a page from the server. More request methods similar to GET are available with HTTP, each performing specific actions on the target resource. Let's learn about these methods and their role in client-server communication over HTTP.

The set of common methods for HTTP/1.1 is listed in the following table:

Method

Description

GET

This method is used for retrieving resources from the server by using the given URI.

HEAD

This method is the same as the GET request, but it only transfers the status line and the header section without the response body.

POST

This method is used for posting data to the server. The server stores the data (entity) as a new subordinate of the resource identified by the URI. If you execute POST multiple times on a resource, it may yield different results.

PUT

This method is used for updating the resource pointed at by the URI. If the URI does not point to an existing resource, the server can create the resource with that URI.

DELETE

This method deletes the resource pointed at by the URI.

TRACE

This method is used for echoing the contents of the received request. This is useful for the debugging purpose with which the client can see what changes (if any) have been made by the intermediate servers.

OPTIONS

This method returns the HTTP methods that the server supports for the specified URI.

CONNECT

This method is used for establishing a connection to the target server over HTTP.

PATCH

This method is used for applying partial modifications to a resource identified by the URI.

We may use some of these HTTP methods, such as GET, POST, PUT, and DELETE, while building RESTful web services in the later chapters.

Continuing our discussion on HTTP, the next section discusses the HTTP header parameter that identifies the content type for the message body.

Representing content types using HTTP header fields

When we discussed the HTTP request-response model in the Understanding the HTTP request-response model section, we talked about the HTTP header parameters (the name-value pairs) that define the operating parameters of an HTTP transaction. In this section, we will cover the header parameter used for describing the content types present in the request and the response message body.

The Content-Type header in an HTTP request or response describes the content type for the message body. The Accept header in the request tells the server the content types that the client is expecting in the response body. The content types are represented using the Internet media type. The Internet media type (also known as the MIME type) indicates the type of data that a file contains. Here is an example:

Content-Type: text/html

This header indicates that the body content is presented in the html format. The format of the content type values is a primary type/subtype followed by an optional semicolon delimited attribute-value pairs (known as parameters).

The Internet media types are broadly classified in to the following categories on the basis of the primary (or initial) Content-Type header:

  • text: This type indicates that the content is plain text and no special software is required to read the contents. The subtype represents more specific details about the content, which can be used by the client for special processing, if any. For instance, Content-Type: text/html indicates that the body content is html, and the client can use this hint to kick off an appropriate rendering engine while displaying the response.

  • multipart: As the name indicates, this type consists of multiple parts of the independent data types. For instance, Content-Type: multipart/form-data is used for submitting forms that contain the files, non-ASCII data, and binary data.

  • message: This type encapsulates more messages. It allows messages to contain other messages or pointers to other messages. For instance, the Content-Type: message/partial content type allows for large messages to be broken up into smaller messages. The full message can then be read by the client (user agent) by putting all the broken messages together.

  • image: This type represents the image data. For instance, Content-Type: image/png indicates that the body content is a .png image.

  • audio: This type indicates the audio data. For instance, Content-Type: audio/mpeg indicates that the body content is MP3 or other MPEG audio.

  • video: This type indicates the video data. For instance, Content-Type: video/mp4 indicates that the body content is MP4 video.

  • application: This type represents the application data or binary data. For instance, Content-Type: application/json; charset=utf-8 designates the content to be in the JavaScript Object Notation (JSON) format, encoded with UTF-8 character encoding.

Tip

JSON is a lightweight data-interchange format. If you are not familiar with the JSON format, not to worry now; we will cover this topic in Chapter 2, Java APIs for JSON Processing.

We may need to use some of these content types in the next chapters while developing the RESTful web services. This hint will be used by the client to correctly process the response body.

Note

We are not covering all the possible subtypes for each category of media type here. To refer to the complete list, visit the website of Internet Assigned Numbers Authority (IANA) at http://www.iana.org/assignments/media-types/media-types.xhtml.

The next topic, a simple but important one, is on HTTP status codes.

HTTP status codes

For every HTTP request, the server returns a status code indicating the processing status of the request. In this section, we will see some of the frequently used HTTP status codes. A basic understanding of status codes will definitely help us later while designing RESTful web services:

  • 1xx Informational: This series of status codes indicates informational content. This means that the request is received and processing is going on. Here are the frequently used informational status codes:

    • 100 Continue: This code indicates that the server has received the request header and the client can now send the body content. In this case, the client first makes a request (with the Expect: 100-continue header) to check whether it can start with a partial request. The server can then respond either with 100 Continue (OK) or 417 Expectation Failed (No) along with an appropriate reason.

    • 101 Switching Protocols: This code indicates that the server is OK for a protocol switch request from the client.

    • 102 Processing: This code is an informational status code used for long running processing to prevent the client from timing out. This tells the client to wait for the future response, which will have the actual response body.

  • 2xx Success: This series of status codes indicates the successful processing of requests. Some of the frequently used status codes in this class are as follows:

    • 200 OK: This code indicates that the request is successful and the response content is returned to the client as appropriate.

    • 201 Created: This code indicates that the request is successful and a new resource is created.

    • 204 No Content: This code indicates that the request is processed successfully, but there's no return value for this request. For instance, you may find such status codes in response to the deletion of a resource.

  • 3xx Redirection: This series of status codes indicates that the client needs to perform further actions to logically end the request. A frequently used status code in this class is as follows:

    • 304 Not Modified: This status indicates that the resource has not been modified since it was last accessed. This code is returned only when allowed by the client via setting the request headers as If-Modified-Since or If-None-Match. The client can take appropriate action on the basis of this status code.

  • 4xx Client Error: This series of status codes indicates an error in processing the request. Some of the frequently used status codes in this class are as follows:

    • 400 Bad Request: This code indicates that the server failed to process the request because of the malformed syntax in the request. The client can try again after correcting the request.

    • 401 Unauthorized: This code indicates that authentication is required for the resource. The client can try again with the appropriate authentication.

    • 403 Forbidden: This code indicates that the server is refusing to respond to the request even if the request is valid. The reason will be listed in the body content if the request is not a HEAD method.

    • 404 Not Found: This code indicates that the requested resource is not found at the location specified in the request.

    • 405 Method Not Allowed: This code indicates that the HTTP method specified in the request is not allowed on the resource identified by the URI.

    • 408 Request Timeout: This code indicates that the client failed to respond within the time frame set on the server.

    • 409 Conflict: This code indicates that the request cannot be completed because it conflicts with some rules established on resources, such as validation failure.

  • 5xx Server Error: This series of status codes indicates server failures while processing a valid request. Here is one of the frequently used status codes in this class:

    • 500 Internal Server Error: This code indicates a generic error message, and it tells that an unexpected error occurred on the server and the request cannot be fulfilled.

    Note

    To refer to the complete list of HTTP status codes maintained by IANA, visit http://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml.

With this topic, we have finished the crash course on HTTP basics. We will be resuming our discussion on RESTful web services in the next section. Take a deep breath and get ready for an exciting journey.

The evolution of RESTful web services

Before getting into the details of REST-enabled web services, let's take a step back and define what a web service is. Then, we will see what makes a web service RESTful.

A web service is one of the very popular methods of communication between the client and server applications over the Internet. In simple words, web services are web application components that can be published, found, and used over the web. Typically, a web service has an interface describing the web service APIs, which is known as Web Services Description Language (WSDL). A WSDL file can be easily processed by machines, which blows out the integration complexities that you may see with large systems. Other systems interact with the web service by using Simple Object Access Protocol (SOAP) messages. The contract for communication is driven by the WSDL exposed by the web service. Typically, communication happens over HTTP with XML in conjunction with other web-related standards.

What kind of problems do the web services solve? There are two main areas where web services are used:

  • Many of the companies specialized in Internet-related services and products have opened their doors to developers using publicly available APIs. For instance, companies such as Google, Yahoo, Amazon, and Facebook are using web services to offer new products that rely on their massive hardware infrastructures. Google and Yahoo offer their search services; Amazon offers its on-demand hosting storage infrastructure and Facebook offers its platform for targeted marketing and advertising campaigns. With the help of web services, these companies have opened the door for the creation of products that did not exist some years ago.

  • Web services are being used within the enterprises to connect previously disjointed departments such as marketing and manufacturing. Each department or line of business (LOB) can expose its business processes as a web service, which can be consumed by the other departments.

    By connecting more than one department to share information by using web services, we begin to enter the territory of the Service-Oriented Architecture (SOA). The SOA is essentially a collection of services, each talking to one another in a well-defined manner, in order to complete relatively large and logically complete business processes.

All these points lead to the fact that a web service has evolved into a powerful and effective channel of communication between a client and a server over a period of time. The good news is that we can integrate RESTful systems into a web service-oriented computing environment without much effort. Although you may have a fair idea about RESTful web services by now, let's see the formal definition before proceeding further.

Note

What is a RESTful web service?

Web services that adhere to the REST architectural constraints are characterized as RESTful web services. Refer to the section, The REST architectural style, at the beginning of this chapter if you need a quick brush up on the architectural constraints for a RESTful system.

Remember that REST is not the system's architecture in itself, but it is a set of constraints that when applied to the system's design leads to a RESTful architecture. As our definition of a web service does not dictate the implementation details of a computing unit, we can easily incorporate RESTful web services to solve large-scale problems. We can even fully use RESTful web services under the larger umbrella of the SOA.

With this larger view of the SOA, we begin to see how REST has the potential to impact the new computing models being developed.

Note

The RESTful web API or REST API is an API implemented using HTTP and the REST architectural constraints. Technically speaking, this is just another term for a RESTful web service. In this book, we will use these terms interchangeably.

 

The core architectural elements of a RESTful system


Having learned the basics of a RESTful system, you are now ready to meet the more exciting concepts of REST. In this section, we will learn the core architectural elements that make a system RESTful.

A uniform interface is fundamental to the architecture of any RESTful system. In plain words, this term refers to a generic interface to manage all interactions between a client and a server in a unified way. All resources (or business data) involved in the client-server interactions are dealt with by a fixed set of operations. The following are core elements that form a uniform interface for a RESTful system:

  • Resources and their identifiers

  • Representations of resources

  • Generic interaction semantics for the REST resources

  • Self-descriptive messages

  • Hypermedia as the engine of an application state

Let's look at these items in detail.

Resources

A RESTful resource is anything that is addressable over the Web. By addressable, we mean resources that can be accessed and transferred between clients and servers. Subsequently, a resource is a logical, temporal mapping to a concept in the problem domain for which we are implementing a solution.

Here are some examples of the REST resources:

  • A news story

  • The temperature in NY at 4:00 p.m. EST

  • A tax return stored in the IRS database

  • A list of code revision history in a repository such as SVN or CVS

  • A student in a classroom in a school

  • A search result for a particular item in a Web index, such as Google

Even though a resource's mapping is unique, different requests for a resource can return the same underlying binary representation stored in the server. For example, let's say we have a resource within the context of a publishing system. Then, a request for the latest revision published and a request for revision number 12 will at some point in time return the same representation of the resource. In this example, the last revision is Version 12. However, when the latest revision published is increased to Version 13, a request to the latest revision will return Version 13, and a request for revision 12 will continue returning Version 12. This implies that in a RESTful architecture, each resource can be accessed directly and independently, and sometimes, different requests may point to the same resource.

As we are using HTTP to communicate, we can transfer a variety of data types between clients and servers as long as the data type used is supported by HTTP. For example, if we request a text file from CNN, our browser receives a text file. If we request a Flash movie from YouTube, our browser receives a Flash movie. The data is streamed in both cases over TCP/IP and the browser knows how to interpret the binary streams because of the Content-Type header present in the HTTP response header. Following this principle, in a RESTful system, the representation of a resource in the response body depends on the desired Internet media type, which is specified within the request header sent by the client.

URI

A URI is a string of characters used to identify a resource over the Web. In simple words, the URI in a RESTful web service is a hyperlink to a resource, and it is the only means for clients and servers to exchange representations.

The client uses a URI to locate the resources over Web and then, sends a request to the server and reads the response. In a RESTful system, the URI is not meant to change over time as it may break the contract between a client and a server. More importantly, even if the underlying infrastructure or hardware changes (for example, swapping the database servers) for a server hosting REST APIs, the URIs for resources are expected to remain the same as long as the web service is up and running.

The representation of resources

The representation of resources is what is sent back and forth between clients and servers in a RESTful system. A representation is a temporal state of the actual data located in some storage device at the time of a request. In general terms, it is a binary stream together with its metadata that describes how the stream is to be consumed by the client. The metadata can also contain extra information about the resource, for example, validation, encryption information, or extra code to be executed at runtime.

Throughout the life of a web service, there may be a variety of clients requesting resources. Different clients can consume different representations of the same resource. Therefore, a representation can take various forms, such as an image, a text file, an XML, or a JSON format. However, all clients will use the same URI with appropriate Accept header values for accessing the same resource in different representations.

For the human-generated requests through a web browser, a representation is typically in the form of an HTML page. For automated requests from the other web services, readability is not as important and a more efficient representation, such as JSON or XML, can be used.

Generic interaction semantics for REST resources

In the previous sections, we introduced the concepts of resources and representations. We learned that resources are mappings of the actual entity states that are exchanged between clients and servers. Further, we discussed that representations are negotiated between clients and servers through the communication protocol (HTTP) at runtime. In this section, we will learn about the generics of interaction semantics and self-descriptive messages followed for the client-server communication in a RESTful system.

Developing RESTful web services is similar to what we have been doing up to this point with our web applications. In a RESTful web service, resources are exchanged between the client and the server, which represent the business entities or data. HTTP specifies methods or actions for the resources. The most commonly used HTTP methods or actions are POST, GET, PUT, and DELETE. This clearly simplifies the REST API design and makes it more readable. On the other hand, in traditional application development, we can have countless actions with no naming or implementation standards. This may call for more development effort for both the client and the server, and make the APIs less readable.

In a RESTful system, we can easily map our CRUD actions on the resources to the appropriate HTTP methods such as POST, GET, PUT, and DELETE. This is shown in the following table:

Data action

HTTP equivalent

CREATE

POST or PUT

READ

GET

UPDATE

PUT or PATCH

DELETE

DELETE

In fact, the preceding list of HTTP methods is incomplete. There are some more HTTP methods available, but they are less frequently used in the context of RESTful implementations. Of these less frequent methods, OPTIONS and HEAD are used more often than others. So, let's glance at these two method types:

  • OPTIONS: This method is used by the client to determine the options or actions associated with the target resource, without causing any action on the resource or retrieval of the resource

  • HEAD: This method can be used for retrieving information about the entity without having the entity itself in the response

In their simplest form, RESTful web services are networked applications that manipulate the state of resources. In this context, resource manipulation means resource creation, retrieval, update, and deletion. However, RESTful web services are not limited to just these four basic data manipulation concepts. They can even be used for executing business logic on the server, but remember that every result must be a resource representation of the domain at hand.

A uniform interface brings all the aforementioned abstractions into focus. Consequently, putting together all these concepts, we can describe RESTful development with one short sentence: we use URIs to connect clients and servers in order to exchange resources in the form of representations.

Let's now look at the four HTTP request types in detail and see how each of them is used to exchange representations to modify the state of resources.

The HTTP GET method

The method, GET, is used to retrieve resources. Before digging into the actual mechanics of the HTTP GET request, we first need to determine what a resource is in the context of our web service and what type of representation we are exchanging.

For the rest of this section, we will use the example of a RESTful web service handling department details for an organization. For this service, the JSON representation of a department looks like the following:

{"departmentId":10,"departmentName":"IT","manager":"John Chen"}

The JSON representation of the list of departments looks like the following:

[{"departmentId":10,"departmentName":"IT","manager":"John Chen"},
{"departmentId":20,"departmentName":"Marketing","manager":"Ameya J"},
{"departmentId":30,"departmentName":"HR","manager":"Pat Fay"}]

With our representations defined, we now assume URIs of the form http://www.packtpub.com/resources/departments to access a list of departments, and http://www.packtpub.com/resources/departments/{name} to access a specific department with a name (unique identifier).

Tip

To keep this example simple and easy to follow, we treat the department name as a unique identifier here. Note that in real life, you can use a server-generated identifier value, which does not repeat across entities, to uniquely identify a resource instance.

We can now begin making requests to our web service. For instance, if we wanted a record for the IT department, we would make a request to the following URI: http://www.packtpub.com/resources/departments/IT.

A representation of the IT department at the time of the request may look like the following code:

{"departmentId":10,"departmentName":"IT","manager":"John Chen"}

Let's have a look at the request details. A request to retrieve details of the IT department uses the GET method with the following URI:

http://www.packtpub.com/resources/departments/IT

Let's see what happens when a client requests for the IT department by using the preceding mentioned URI. Here is the sequence diagram for the GET request:

What is happening here?

  1. A Java client makes an HTTP request with the GET method type and IT as the identifier for the department.

  2. The client sets the representation type that it can handle through the Accept request header field. This request message is self-descriptive:

    • It uses a standard method (the GET method in this example) with known semantics for retrieving the content

    • The content type is set to a well-known media type (text/plain)

    • This request also declares the acceptable response format

  3. The web server receives and interprets the GET request to be a retrieve action. At this point, the web server passes control to the underlying RESTful framework to handle the request. Note that RESTful frameworks do not automatically retrieve resources, as that is not their job. The job of a framework is to ease the implementation of the REST constraints. Business logic and storage implementation is the role of the domain-specific Java code.

  4. The server-side program looks for the IT resource. Finding the resource could mean looking for it in some data store such as a database, a file system, or even a call to a different web service.

  5. Once the program finds the IT department details, it converts the binary data of the resource to the client's requested representation. In this example, we use the JSON representation for the resource.

  6. With the representation converted to JSON, the server sends back an HTTP response with a numeric code of 200 together with the JSON representation as the payload. Note that if there are any errors, the HTTP server reports back the proper numeric code, but it is up to the client to correctly deal with the failure. Similar to the request message, the response is also self-descriptive.

All the messages between a client and a server are standard HTTP calls. For every retrieve action, we send a GET request and we get an HTTP response back with the payload of the response being the representation of the resource or, if there is a failure, a corresponding HTTP error code (for example, 404, if a resource is not found; 500, if there is a problem with the Java code in the form of an exception).

Getting a representation for all departments works the same way as getting a representation for a single department, although we now use the URI as http://www.packtpub.com/resources/departments and the result is the JSON representation, which looks like the following code:

[{"departmentId":10,"departmentName":"IT","manager":"John Chen"},
{"departmentId":20,"departmentName":"Marketing","manager":"Ameya J"},
{"departmentId":30,"departmentName":"HR","manager":"Pat Fay"}]

Note

The HTTP GET method should only be used to retrieve representations, not for performing any update on the resource. A GET request must be safe and idempotent. For more information, refer to http://www.w3.org/DesignIssues/Axioms.

For a request to be safe, it means that multiple requests to the same resource do not change the state of the data in the server. Assume that we have a representation, R, and requests happen at a time, t. Then, a request at time t1 for resource R returns R1; subsequently, a request at time t2 for resource R returns R2 provided that no further update actions have been taken between t1 and t2. Then, R1 = R2 = R.

For a request to be idempotent, multiple calls to the same action should not change the state of the resource. For example, multiple calls to create resource R at times t1, t2, and t3 mean that R will exist only as R and the calls at times t2 and t3 are ignored.

The HTTP POST method

The POST method is used to create resources. As we are creating a department, we use the HTTP POST method. Again, the URI to create a new department in our example is http://www.packtpub.com/resources/departments. The method type for the request is set by the client.

Assume that the Sales department does not exist in our list, and we want to add it to the list. The Sales data representation looks like the following:

{"departmentName":"Sales","manager":"Tony Greig"}

Now, the sequence diagram of our POST request looks like the following:

The series of steps for the POST request is as follows:

  1. A Java client makes a request to the http://www.packtpub.com/resources/departments URI with the HTTP method set to POST.

  2. The POST request carries the payload along with it in the form of a JSON representation of the Sales department.

  3. The server receives the request and lets the REST framework handle it; our code within the framework executes the proper commands to store the representation, irrespective of which data persistence mechanism is used.

  4. Once the new resource is stored, a response code, 2xx (representing a successful operation), is sent back. In this example, the server sends 201 Created, which implies that the server has fulfilled the request by creating a new resource. The newly created resource is accessible by traversing the URI given by a Location header field. If it is a failure, the server sends the appropriate error code.

The HTTP PUT method

The PUT method is used to update resources. To update a resource, we first need its representation in the client; secondly, at the client level, we update the resource with the new value(s) that we want; and finally, we update the resource by using a PUT request together with the representation as its payload.

In this example, let's add a manger to the Sales department that we created in the previous example.

Our original representation of the Sales department is as follows:

{"departmentId":40,"departmentName":"Sales","manager":"Tony Greig" }

Let's update the manager for the Sales department; our representation is as follows:

{"departmentId":40,"departmentName":"Sales","manager":"Ki Gee"}

We are now ready to connect to our web service to update the Sales department by sending the PUT request to http://www.packtpub.com/resources/departments/Sales. The sequence diagram of our PUT request is as follows:

The series of steps for the PUT request is as follows:

  1. A Java client makes a PUT request to http://www.packtpub.com/resources/departments/Sales with the JSON payload representing the modified department details.

  2. The server receives the request and lets the REST framework handle it. At this point, we let our code execute the proper commands to update the representation of the Sales department. Once completed, a response is sent back. The 204 No Content response code indicates that the server has fulfilled the request but does not return the entity body.

The HTTP DELETE method

The DELETE method is used to delete the resource. In this example, we will delete a resource by making use of the same URI that we used in the other three cases.

Assume that we want to delete the Sales department from the data storage. We send a DELETE request to our service with the following URI: http://www.packtpub.com/resources/departments/Sales.

The sequence diagram for our DELETE request is shown in the following diagram:

The series of steps for the DELETE request is as follows:

  1. A Java client makes a DELETE request to http://www.packtpub.com/resources/departments/Sales.

  2. The server receives the request and lets the REST framework handle it. At this point, the server code executes the proper commands to delete the representation of the Sales department.

  3. Once completed, a response is sent back.

With this, we have covered all the major actions that can be carried out on resources in a RESTful web service. To keep things simple during our discussion, we did not talk about the actual implementation of the CREATE, READ, UPDATE, and DELETE operations on the resource. In all three examples, we presumed that we have a well-behaved web service that adheres to the RESTful guidelines and the client and the server communicate over HTTP. We use the communication protocol to send action requests, and our resource representations are sent back and forth through unchanging URIs. We will cover more detailed end-to-end examples later in this book.

Note

A point to note about our sequence diagrams is that we are assuming that all the underlying technologies are Java technologies (servers and clients). However, these are just components in the whole architecture and the explanations apply to any technology stack.

Hypermedia as the Engine of Application State

Hypermedia as the Engine of Application State (HATEOAS) is an important principle of the REST application architecture. The principle is that the model of application changes from one state to another by traversing the hyperlinks present in the current set of resource representations (the model). Let's learn this principle in detail.

In a RESTful system, there is no fixed interface between the client and the server as you may see in a conventional client-server communication model such as Common Object Request Broker Architecture (CORBA) and Java Remote Method Invocation (Java RMI). With REST, the client just needs to know how to deal with the hypermedia links present in the response body; next, the call to retrieve the appropriate resource representation is made by using these dynamic media links. This concept makes the client-server interaction very dynamic and keeps it different from the other network application architectures.

Here is an example illustrating the HATEOAS principle. In this example, the http://www.packtpub.com/resources/departments/IT URI returns the following response to the client:

{"departmentId":10,
"departmentName":"IT",
"manager":"John Chen,
"links": [ {
        "rel": "employees",
        "href": "http://packtpub.com/resources/departments/IT/employees"
    } ]"}

This is the current state of the system. Now, to get the employees belonging to the department, the client traverses the hyperlink present in the response body, namely http://www.packtpub.com/resources/departments/IT/employees. This URI returns the following employee list. The application state now changes into the following form (as represented by the response content):

[{"employeeId":100,
 "firstName":"Steven",
 "lastName":"King",
 "links": [ {
        "rel": "self",
        "href": "http://www.packtpub.com/resources/employees/100"
    }]
 },
{"employeeId":101,
"firstName":"Neena",
"lastName":"Kochhar",
"links": [ {
        "rel": "self",
        "href": "http://www.packtpub.com/resources/employees/101"
     }]
 }]

In this example, the application state changes from one state to another when the client traverses the hypermedia link. Hence, we refer to this implementation principle as Hypermedia as the Engine of Application State.

 

Description and discovery of RESTful web services


As you may know, WSDL is used for describing the functionality offered by a SOAP web service. For a SOAP web service, this is a widely accepted standard and is supported by many enterprises today. In contrast, for RESTful web services, there is no such standard and you may find different metadata formats used by various enterprises.

However, in general, you may see the following goals in common among all these metadata formats for RESTful APIs, although they differ in their syntax and semantics:

  • Entry points for the service

  • Resource paths for accessing each resource

  • HTTP methods allowed to access these resources, such as GET, POST, PUT, and DELETE

  • Additional parameters that need to be supplied with these methods, such as pagination parameters, while reading large collections

  • Format types used for representing the request and response body contents such as JSON, XML, and TEXT

  • Status codes and error messages returned by the APIs

  • Human readable documentation for REST APIs, which includes the documentation of the request methods, input and output parameters, response codes (success or error), API security, and business logic

Some of the popular metadata formats used for describing REST APIs are Web Application Description Language (WADL), Swagger, RESTful API Modeling Language (RAML), API Blueprint, and WSDL 2.0. We will have a more detailed discussion on each of these items in Chapter 7, The Description and Discovery of RESTful Web Services.

 

Java tools and frameworks for building RESTful web services


Over the last few years, the REST architectural style has become very popular in the industry and many enterprises have accepted it as the current standard for building public web APIs, particularly when scalability and simplicity are major concerns for them. Today, one may see many tools and frameworks available on the market for building RESTful web services. In this section, we will briefly discuss some of the most popular Java-based frameworks and tools for building RESTful systems.

The Java API for RESTful web services (JAX-RS) is the Java API for creating RESTful web services following the REST architectural pattern discussed in this chapter. JAX-RS is a part of the Java Platform Enterprise Edition (Java EE) platform and is designed to be a standard and portable solution. There are many reference implementations available for JAX-RS today. Some of the most popular implementations are Jersey, Apache CXF, RESTEasy, and Restlet. At this juncture, it is worth mentioning that most of the frameworks in the preceding list, such as Jersey and Apache CXF, are not just limited to reference implementations of the JAX-RS specifications, but they also offer many additional features on top of the specifications.

Apart from the JAX-RS-based frameworks (or extensions built on top of JAX-RS), you may also find some promising nonstandard (not based on JAX-RS) Java REST frameworks on the market. Some such frameworks are as follows:

  • One such framework is RESTX, which is an open source Java REST framework and is primarily focused on the server-side REST API development. This is relatively new on the market and simplifies the REST API development.

  • Spark is another framework that falls into this category. It is a Java web framework with support for building REST APIs. Spark 2.0 is built using Java 8, leveraging all the latest improvements of the Java language.

  • Play is another framework worth mentioning in this category. It is a Java (and Scala)-based web application framework with inherent support for building RESTful web services.

Discussing all these frameworks does not come under the scope of this book. We will focus on some of the most popular frameworks with many real-life use cases in mind. In the next chapters, you will learn JAX-RS as well as additional features available with the Jersey framework for building scalable and maintainable RESTful web services.

 

Summary


This chapter is intended to give an overview of RESTful web services. This is essential for an easy understanding of what we will learn in the rest of the book. As we have just started our topic, we have not covered any code samples in this chapter to keep it simple. In the following chapters, we will examine the most popular Java tools and frameworks available for building a RESTful web service along with many real-life examples and code samples.

In the next chapter, we will discuss the JSON representation of the REST resources and the Java APIs for JSON processing.

About the Author

  • Jobinesh Purushothaman

    Jobinesh Purushothaman works with Oracle as a consulting solutions architect. In his current role, he is involved in the design and architectural decisions of various products that use Java EE and Oracle Application Development Framework technologies. Occasionally, he speaks at industry conferences, such as JavaOne and the Oracle Technology Network Developer Day. Jobinesh authored his first book, Oracle ADF Real World Developer's Guide, in 2012 for Packt Publishing. You can find his blog at http://www.jobinesh.com.

    He holds a master of science (MSc) degree in computer science from Bharathiar University, India, and a master of business administration (MBA) degree from Indira Gandhi National Open University (IGNOU), India. After completing his MSc in computer science, Jobinesh started his career in 1999 with MicroObjects Private Limited, India. His career has taken him to different countries and various companies, where he has worked as a developer, technical leader, mentor, and technical architect. Jobinesh joined Oracle India Private Limited in 2008. Prior to joining Oracle, he worked as a senior software engineer at Emirates Group IT, Dubai from 2004 to 2008, where he was part of an IT strategy and architecture team.

    Jobinesh currently lives in Bangalore, India, with his wife, Remya, son, Chinmay, and daughter, Ameya.

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very good book it cover every thing i need .
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