How to use the command 'java_home' (with examples)

Java_home is a command that allows you to return the value for $JAVA_HOME or execute a command using this variable. It is commonly used in Unix operating systems and provides information about Java Virtual Machines (JVMs) installed on a system.

Use case 1: List JVMs based on a specific version

Code:

java_home --version 1.5+

Motivation: By listing JVMs based on a specific version, you can easily identify which JVMs are compatible with a particular Java version. This is useful for developers who need to ensure their application runs on a specific JVM version or for system administrators who want to verify the compatibility of JVMs installed on their servers.

Explanation:

• java_home: This is the command to be executed.
• --version 1.5+: This argument specifies the Java version to filter JVMs. In this example, it searches for JVMs compatible with Java 1.5 and higher.

Example output:

/Library/Java/JavaVirtualMachines/jdk1.8.0_291.jdk/Contents/Home
/Library/Java/JavaVirtualMachines/jdk-11.0.12.jdk/Contents/Home

Use case 2: List JVMs based on a specific architecture

Code:

java_home --arch i386

Motivation: Sometimes it is necessary to identify JVMs based on a specific architecture, especially when dealing with systems that support multiple architectures. This can help in scenarios where specific JVM features or optimizations are needed for a particular architecture.

Explanation:

• java_home: This is the command to be executed.
• --arch i386: This argument specifies the architecture to filter JVMs. In this example, it searches for JVMs with the architecture ‘i386’, which is commonly used for 32-bit Intel processors.

Example output:

/Library/Java/JavaVirtualMachines/jdk1.8.0_291.jdk/Contents/Home

Use case 3: List JVMs based on specific tasks

Code:

java_home --datamodel Applets|WebStart|BundledApp|JNI|CommandLine

Motivation: Different tasks may require different JVM configurations. By listing JVMs based on specific tasks, you can easily identify JVMs with the necessary configurations for a particular use case. This can be helpful for developers or system administrators who need to ensure their Java applications or services are using the correct JVM settings.

Explanation:

• java_home: This is the command to be executed.
• --datamodel Applets|WebStart|BundledApp|JNI|CommandLine: This argument specifies the specific tasks (separated by ‘|’) to filter JVMs. In this example, it searches for JVMs configured for Applets, WebStart, BundledApp, JNI, or CommandLine tasks.

Example output:

/Applications/JavaApplets.plugin/Contents/Home
/Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home
/Library/Java/JavaVirtualMachines/jdk1.8.0_291.jdk/Contents/Home

(Note: The output might vary based on the installed JVMs and system configuration)

Use case 4: List JVMs in a XML format

Code:

java_home --xml

Motivation: Listing JVMs in XML format can be helpful for automated processing or integrating with other tools that require an XML-based input. This can simplify parsing and extracting information from the JVM list.

Explanation:

• java_home: This is the command to be executed.
• --xml: This argument specifies to output the JVM list in XML format.

Example output:

<array>
    <string>/Library/Java/JavaVirtualMachines/jdk1.8.0_291.jdk/Contents/Home</string>
    <string>/Library/Java/JavaVirtualMachines/jdk-11.0.12.jdk/Contents/Home</string>
</array>

Use case 5: Display help

Code:

java_home --help

Motivation: When learning a new command or needing a quick reference on its usage, displaying the help documentation is essential. It provides an overview of available options, arguments, and their meanings.

Explanation:

• java_home: This is the command to be executed.
• --help: This argument requests the command to display help documentation.

Example output:

Usage: java_home [--full-version] [--version] [--exec] [--failfast] [--datamodel <type>] [--arch <arch>] [--copy] [--proxy] [--xml] [--request] [--pid <pid>] [--vmext <ext>] [--jre <jre_home>] [--request] [--requestjdk] [--dev] [--mainclass <class_name>]
   --full-version, -F
    Print Java Development Kit full version rather than version number.
    Default: false
   --version, -V
    Print only the version number.
    Default: false
   --exec, -e
    Execute command and return the path to the Java home directory relative to JAVA_HOME.
    Default: false
   --failfast, -f
    Fail with status 1 if no api supports requested

...

Conclusion:

The java_home command is a useful tool for managing and discovering JVMs on a Unix system. By using various options and arguments, you can filter, manipulate, and retrieve JVM information in different formats. Whether you need to find JVMs based on a specific version, architecture, or tasks, or retrieve their paths in XML format, java_home provides the necessary capabilities for such use cases. Furthermore, the command also offers a help option to guide users on its usage, making it an effective tool for JVM administration and development purposes.