Java 11 Cookbook - Second Edition

Now that we have finished installing JDK, let's see how we can set the PATH variable.

The tools provided with JDK, namely javac, java, jconsole, and jlink, are available in the bin directory of your JDK installation. There are two ways you could run these tools from the Command Prompt:

      cd "C:\Program Files\Java\jdk-11\bin"
      javac -version

Let's see how you can add the JDK bin directory to the PATH variable:

The variables defined under System variables are available across all the users of the system, and those defined under User variables for <user name> are available only to the specific user.

The following screenshot shows the other Windows versions:

      $> vim ~/.bash_aliases
      export JAVA_HOME=/usr/lib/jdk-11
      export PATH=$PATH:$JAVA_HOME/bin

Source the .bashrc file to apply the new aliases:

      $> source ~/.bashrc
      $> echo $JAVA_HOME
      /usr/lib/jdk-11
      $>javac -version
      javac 11-ea
      $> java -version
      java version "11-ea" 2018-09-25
      Java(TM) SE Runtime Environment 18.9 (build 11-ea+22)
      Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11-ea+22, mixed 
      mode)

You should have JDK installed and the PATH variable updated to point to the JDK installation.

        @XmlRootElement
        @XmlAccessorType(XmlAccessType.FIELD) 
        class Messages{     
          @XmlElement 
          public final String message = "Hello World in XML"; 
        }

In the preceding code, @XmlRootElement is used to define the root tag, @XmlAccessorType is used to define the type of source for the tag name and tag values, and @XmlElement is used to identify the sources that become the tag name and tag values in the XML.

public class HelloWorldXml{
  public static void main(String[] args) throws JAXBException{
    JAXBContext jaxb = JAXBContext.newInstance(Messages.class);
    Marshaller marshaller = jaxb.createMarshaller();
    marshaller.setProperty(Marshaller.JAXB_FRAGMENT,Boolean.TRUE);
    StringWriter writer = new StringWriter();
    marshaller.marshal(new Messages(), writer);
    System.out.println(writer.toString());
  } 
}

    module com.packt{
      //depends on the java.xml.bind module
      requires java.xml.bind;
      //need this for Messages class to be available to java.xml.bind
      exports  com.packt to java.xml.bind;
    }

The directory structure with the preceding files is as follows:

 mkdir -p mods/com.packt

Compile the source, HelloWorldXml.java and module-info.java, into the mods/com.packt directory:

      javac -d mods/com.packt/ src/com.packt/module-info.java
      src/com.packt/com/packt/HelloWorldXml.java

<messages><message>Hello World in XML</message></messages>

Don't worry if you are not able to understand the options passed with the java or javac commands. You will learn about them in Chapter 3, Modular Programming. 

The Java 10 release (JDK 18.3) started a six-month release cycle—every March and every September—and a new release numbering system. It also introduced many new features, the most significant of which (for application developers) are the following:

See the full list of Java 10's new features in the release notes: https://www.oracle.com/technetwork/java/javase/10-relnote-issues-4108729.html.

We will discuss the new features of JDK 18.9 in more detail in the next section.

We have picked a few features that we feel are the most important and useful for an application developer.

var s = Character.toString(50);
System.out.println(s);  //prints: 2

var i = CharSequence.compare("a", "b");
System.out.println(i);   //prints: -1

i = CharSequence.compare("b", "a");
System.out.println(i);   //prints: 1

i = CharSequence.compare("this", "that");
System.out.println(i);   //prints: 8

i = CharSequence.compare("that", "this");
System.out.println(i);   //prints: -8

String s1 = "a";
String s2 = s1.repeat(3); //prints: aaa
System.out.println(s2);

String s3 = "bar".repeat(3);
System.out.println(s3); //prints: barbarbar

String s1 = "a";
System.out.println(s1.isBlank());  //false
System.out.println(s1.isEmpty());  //false

String s2 = "";
System.out.println(s2.isBlank());  //true
System.out.println(s2.isEmpty());  //true

String s3 = "  ";
System.out.println(s3.isBlank());  //true
System.out.println(s3.isEmpty());  //false

String s = "l1 \nl2 \rl3 \r\nl4 ";
s.lines().forEach(System.out::print); //prints: l1 l2 l3 l4 

String s = " a b ";
System.out.println("'" + s.strip() + "'");        // 'a b'
System.out.println("'" + s.stripLeading() + "'"); // 'a b '
System.out.println("'" + s.stripTrailing() + "'");// ' a b'

Path filePath = Path.of("a", "b", "c.txt");
System.out.println(filePath);     //prints: a/b/c.txt

try {
    filePath = Path.of(new URI("file:/a/b/c.txt"));
System.out.println(filePath);  //prints: /a/b/c.txt
} catch (URISyntaxException e) {
    e.printStackTrace();
}

Pattern pattern = Pattern.compile("^a.*z$");
Predicate<String> predicate = pattern.asMatchPredicate();
System.out.println(predicate.test("abbbbz")); // true
System.out.println(predicate.test("babbbz")); // false
System.out.println(predicate.test("abbbbx")); // false

There are quite a few other changes  introduced in JDK 18.9:

Read the Java 11 (JDK 18.9) release notes for more details and other changes.

The advantages of loading classes from the shared archive became possible for two reasons:

The new JVM functionality allows us to create a list of classes to be shared, then use this list to create a shared archive, and use the shared archive to fast-load archived classes into memory.

java -Xshare:dump

It will create the shared archive as a classes.jsa file. On a Linux system, this file is placed in the following folder:

/Library/Java/JavaVirtualMachines/jdk-11.jdk/Contents/Home/lib/server

On a Windows system, it is placed in the following folder:

C:\Program Files\Java\jdk-11\bin\server

If this folder is accessible by the system admin only, run the command as an admin.

Please notice that not all classes can be shared. For example, the .class files located in the directory on the classpath and classes loaded by custom class loaders cannot be added to the shared archive. 

java -Xshare:on -jar app.jar

The preceding command maps the content of the archive at a fixed address. This memory-mapping operation may occasionally fail when the required address space is not available. If that happens when the -Xshare:on option is used, the JVM exits with an error. Alternatively, the -Xshare:auto option can be used, which just disables the feature and loads the classes from the classpath if the shared archive cannot be used for whatever reason.

java -XX:+UseAppCDS -XX:DumpLoadedClassList=classes.txt -jar app.jar

The preceding command records all the loaded classes in the classes.txt file. If you would like to make your application load faster, stop the JVM just after the application has been started. If you need it to load certain classes faster but these classes are not loaded at the application startup automatically, make sure that the use cases that require these classes are executed.

java -XX:+UseAppCDS -Xshare:dump -XX:SharedClassListFile=classes.txt -XX:SharedArchiveFile=app-shared.jsa --class-path app.jar

Notice that the shared archive file has a name other than classes.jsa, so the default shared archive is not overwritten.

java -XX:+UseAppCDS -Xshare:on -XX:SharedArchiveFile=app-shared.jsa -jar app.jar

Again, you can use the -Xshare:auto option to avoid an unexpected exit of the JVM.

The effect of the shared archive usage depends on the number of classes in it and other particulars of the application. So, we recommend you experiment and test various configurations before committing to a certain list of classes in production.