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1. Introduction to EJDK

The EJDK (Embedded Java Development Kit) was introduced by Oracle to solve the problem of providing binaries for all the available embedded platforms. We can download the latest EJDK from Oracle’s site here.

Simply put, it contains the tools for creating platform-specific JREs.

2. jrecreate

EJDK provides jrecreate.bat for Windows and jrecreate.sh for Unix/Linux platforms. This tool helps in assembling custom JREs for platforms we wish to use, and was introduced to:

  • minimize the release of binaries by Oracle for every platform
  • make it easy to create customized JREs for other platforms

The following syntax is used to execute the jrecreate command; in Unix/Linux:

$jrecreate.sh -<option>/--<option> <argument-if-any>

And in Windows:

$jrecreate.bat -<option>/--<option> <argument-if-any>

Note, we can add multiple options for a single JRE creation. Now, let’s take a look at some of the options available for the tool.

3. Options for jrecreate

3.1. Destination

The destination option is required and specifies the directory in which the target JRE should be created:

$jrecreate.sh -d /SampleJRE

On running the above command, a default JRE will be created in the specified location. The command line output will be:

Building JRE using Options {
    ejdk-home: /installDir/ejdk1.8.0/bin/..
    dest: /SampleJRE
    target: linux_i586
    vm: all
    runtime: jre
    debug: false
    keep-debug-info: false
    no-compression: false
    dry-run: false
    verbose: false
    extension: []
}

Target JRE Size is 55,205 KB (on disk usage may be greater).
Embedded JRE created successfully

From the above result, we can see that the target JRE is created in the specified destination directory. All the other options have taken their default values.

3.2. Profiles

The profile option is used to manage the size of the target JRE. The profiles define the functionality of the API to be included. If the profile option is not specified, the tool will include all the JRE APIs by default:

$jrecreate.sh -d /SampleJRECompact1/ -p compact1

A JRE with a compact1 profile will be created. We can also use ––profile instead of -p in the command. The command line output will display the following result:

Building JRE using Options {
    ejdk-home: /installDir/ejdk1.8.0/bin/..
    dest: /SampleJRECompact1
    target: linux_i586
    vm: minimal
    runtime: compact1 profile
    debug: false
    keep-debug-info: false
    no-compression: false
    dry-run: false
    verbose: false
    extension: []
}

Target JRE Size is 10,808 KB (on disk usage may be greater).
Embedded JRE created successfully

In the above result, note that the runtime option has the value as compact1. Also note the size of the result JRE is just under 11MB, down from 55MB in the previous example.

There are three available options for the profile setting: compact1, compact2, and compact3.

3.3. JVMs

The jvm option is used to customize our target JRE with specific JVMs based on the user’s needs. By default, it includes all the available JVMs (client, server, and minimal) if both profile and jvm options are not specified:

$jrecreate.sh -d /SampleJREClientJVM/ --vm client

A JRE with a client jvm will be created. The command line output will display the following result:

Building JRE using Options {
    ejdk-home: /installDir/ejdk1.8.0/bin/..
    dest: /SampleJREClientJVM
    target: linux_i586
    vm: Client
    runtime: jre
    debug: false
    keep-debug-info: false
    no-compression: false
    dry-run: false
    verbose: false
    extension: []
}

Target JRE Size is 46,217 KB (on disk usage may be greater).
Embedded JRE created successfully

In the above result, note that the vm option has the value Client. We can also specify the other JVMs like server and minimal with this option.

3.4. Extension

The extension option is used to include various allowed extensions to the target JRE. By default, there are no extensions added:

$jrecreate.sh -d /SampleJRESunecExt/ -x sunec

A JRE with an extension sunec (Security provider for Elliptic Curve Cryptography) will be created. We can also use ––extension instead of -x in the command. The command line output will display the following result:

Building JRE using Options {
    ejdk-home: /installDir/ejdk1.8.0/bin/..
    dest: /SampleJRESunecExt
    target: linux_i586
    vm: all
    runtime: jre
    debug: false
    keep-debug-info: false
    no-compression: false
    dry-run: false
    verbose: false
    extension: [sunec]
}

Target JRE Size is 55,462 KB (on disk usage may be greater).
Embedded JRE created successfully

In the above result, note that the extension option has the value sunec. Multiple extensions can be added with this option.

3.5. Other Options

Other than the major options discussed above, jrecreate also facilitates users with a few more options:

  • ––help: displays summary of command line options for jrecreate tool
  • ––debug: creates JRE that has debug support
  • ––keep-debug-info: keeps the debug information from class and unsigned JAR files
  • ––dry-run: performs a dry run without actually creating the JRE
  • ––no-compression: creates a JRE with unsigned JAR files in uncompressed format
  • ––verbose: displays verbose output for all jrecreate commands

4. Conclusion

In this tutorial, we learned the basics of EJDK, and how the jrecreate tool is used to generate platform-specific JREs.

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