Java Developer Guide

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Before going into details of how Java is handled on Gentoo, please make sure the Gentoo Java Packing Policy and also the Java articles have been reviewed.

This guide covers specific details on Gentoo Java ebuilds.

Java on Gentoo

This section will give you more insight into how Gentoo handles Java. You should be familiar with the Java User Guide before proceeding.

Virtual machines (VMs)

As discussed in the User Guide, there are several VMs available from portage.

Testing all packages to ensure they build and run with every VM is a huge undertaking, and there simply are not enough resources to guarantee every package will build and run with every VM.

We now maintain a list of "supported virtual machines" for each architecture. These are the VMs we will test the packages against before committing changes to portage. When you emerge a package, it will by default try to use the best "supported virtual machine."

Of course, Gentoo and Linux in general are about choice, so if you prefer a different VM over the "supported virtual machines", you can easily use that VM instead. However, you should also know that bugs reported with one of the non-"supported virtual machine" will get a low priority if it isn't present using a "supported virtual machine".

Configuring which VM to use

You can choose which VM to use on a per-user basis, and which VM to use for the system (ie when running things as root). Both of these are configured using java-config.

A user's current VM is represented by a symlink located at ~/.gentoo/java-config-2/current-user-vm. This symlink points to the JAVA_HOME of the chosen VM. Similarly, the system VM is represented by a symlink at /etc/java-config-2/current-system-vm.

The current VM can also be changed on the fly. This can be accomplished setting the environment GENTOO_VM to contain the name of a VM that java-config knows about.

Java tools

The traditional Java tools, ie, java, javac, javadoc, etc, are all located in /usr/bin. They are actually all symlinks to the run-java-tool script. This script will call the appropriate tool, depending on how the script is invoked, from the current VM. The contents of the GENTOO_VM variable is checked first, then the user VM, and lastly the system VM.

Build-time VM switching

As outlined in the User Guide, and mentioned briefly earlier, the VM will switch at build time to accommodate the needs of the package. The VM to use is first determined by JAVA_PKG_FORCE_VM, then /etc/java-config-2/build/jdk.conf, and lastly the system VM.

Bytecode compatibility

The default behavior of javac is to compile bytecode that will be compatible with the current VM version and higher (IE forward compatible). It is possible to specify which VM to compile for to provide the best compatibility.

At build time, the DEPEND and RDEPEND variables will determine what VM to compile for based on virtual/jdk and virtual/jre. Additionally, this can be controlled by the environment variables JAVA_PKG_WANT_SOURCE and JAVA_PKG_WANT_TARGET.

There is a wrapper for javac called ejavac, which will use the appropriate VM's javac, and then specify the appropriate -target and -source. For projects that use ant, the build.xml can be patched to specify the appropriate -target and -source.

Filesystem layout

In general, the directory policies are handled for you by the helper functions in the java-utils-2 eclass.

These functions adhere to the following path name conventions:

  • /usr/share/${PN}-${SLOT}/package.env contains information about the package.
  • .jar files created from source code are installed to /usr/share/${PN}-${SLOT}/lib/
  • .jar pre-built files not compiled by the ebuild are installed to /opt/${PN}-${SLOT}/lib
  • Javadoc documentation is installed to /usr/share/doc/${PF}/html/api/
  • Source archives are installed to /usr/share/${PN}-${SLOT}/source/
  • User-executable scripts are installed to /usr/bin
  • System-wide environment files are installed to /usr/env.d/java
  • User-specific environment files can be put into ${HOME}/.gentoo/env.d/

Prepare Enviroment

Before getting started there are a few things to prepare in your development environment, such as environment variables to set, and/or packages to merge.


In addition to ECLASS_DEBUG_OUTPUT=on, there are a couple Java specific portage environment variables to be set. They are optional, but the first is recommend to be set by anyone doing Java ebuild development.

Set this variable in make.conf to enable use of dev-java/java-dep-check. Which will look for and report any unneeded dependencies and/or any bundled dependencies found not part of the package dependencies.
Set this variable to enable debugging in Java eclasses


There are some packages that can help you with Java ebuild development and bumping to new versions.

Java API check

Each time you are bumping a Java library, you should check whether the API of the library did not change significantly. If the change is significant, you should decide whether new SLOT should be introduced to prevent breaking of existing applications depending on this package and the current SLOT.

There are several tools that could help you to get fast overview of the changes.

This package checks for changes in Java API of two jar files. It is fast and also works with latest Java bytecode.
This is a more general tool that can also check for Java API changes. The output is more detailed and also includes information about other files included. It takes more time to generate the report though.

General guidelines

In addition to standard Gentoo ebuild guidelines, there are some specific guidelines for Java packages:

  • Avoid using bundled .jar files at all costs for source-based packages. Instead, they should use system installed versions with the help of our eclass functions.
  • If you only need the path to installed libraries, you can use java-pkg_getjar(s). Don't call java-config directly - it will not record the dependency in the package env.
  • Always provide an easily understandable reason after die, so that end-users will provide the maintainers with sensible feedback if the build should fail.
  • Avoid cluttering up the environment by adding environment files to /etc/env.d/. Instead, store your env file in /etc/env.d/java/, and then have user scripts source their environment file when it launches. Otherwise, developers, who regularly override CLASSPATH, CATALINA_HOME and other environment variables, will have problems running regular apps. If you use the launcher it will also automatically source the appropriate env file.
  • Make sure you always compile with a correct source/target version. This is important to ensure future and backwards compatibility. If the packages use ant, this can be done for you automatically. See java-ant-2.eclass. If not you will have to patch it to pass $(java-pkg_javac-args) to javac. Also, as the lowest supported version of JDK in Gentoo is 1.8, use at least that version if possible to avoid future problems when lower source/target versions will become unsupported by JDKs, which would cause an unnecessary extra maintenance of the packages.
  • Do no install jars which contain versions in their filename, ie castor-0.9.7.jar. Use java-pkg_newjar which renames and installs jars without version in the file name.
  • For packages that use Ant to build, try to DEPEND on just dev-java/ant. dev-java/ant is automatically added to DEPEND if you inherit java-ant-2.eclass . If the package makes use of 'optional' ant tasks, you'll need to DEPEND on dev-java/ant. see the Ant Guide for details.

USE flags

Gentoo Java ebuilds unlike most others start with a USE flag variable, a Java specific one JAVA_PKG_IUSE. The only thing that can come before this variable in an ebuild is the EAPI variable. You still set the other USE flag for various general USE flags, but there are Java specific USE flags that go in this variable and DO NOT go in the other normal USE flag.

About USE flags

For more information regarding USE flags, refer to the USE flags chapter from the Gentoo Development Guide.

Java specific USE flags

There are a few specific common USE flags for Java ebuilds as follows. These use flags do not go in the normal USE variable but go in JAVA_PKG_IUSE instead. Any use flag other than the following would go in the normal USE variable. The JAVA_PKG_IUSE must precede the inherit line in an ebuild.

The USE flags that go in JAVA_PKG_IUSE

  • If USE FLAG binary exists and is set, it will just copy ${JAVA_BINJAR_FILENAME} to ${S} and skip the rest of src_compile.
  • The doc flag will build API documentation using javadoc.
  • The source flag installs a zip of the source code of a package. This is traditionally used for IDEs to 'attach' source to the libraries that are being use;
  • The test Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)

Masked gentoo-vm

The USE flag gentoo-vm is experimental and currently not needed for regular packages. It is only required to run experiments and forcing eselect to also list newer JVMs. See bug #805008 for details on how to unmask this USE flag.

Java USE flag usage

The JAVA_PKG_IUSE USE flag/variable MUST reside in the ebuild before the inherit line.

If a manual or other extensive documentation is available, it should be installed using the doc USE flag. If the build system can, you should build javadocs also using the doc USE flag. If it does not, you should consider patching it to do so, and provide the patch upstream. HTML documentation should be installed using dohtml and javadocs using java-pkg_dojavadoc.

If you want to go all the way, add the source USE flag that installs the complete source code as a .zip file. Use java-pkg_dosrc for this purpose. This allows IDEs such as Eclipse and NetBeans to do complete source-level debugging. The source USE flag will add app-arch/zip to DEPEND automatically, you do not need to add this dependency.

CODE Example of setting common Java USE flags
JAVA_PKG_IUSE="doc source test"

inherit ...


Gentoo Java ebuild dependencies for the most part are just like in any other ebuild. There are two common dependencies that will always exist for any Java ebuild, no matter if Java is optional or not. Though if Java is optional, the dependencies would be conditional based on a USE flag, likely java USE flag. There are some packages which may require a JDK at runtime, otherwise most should have a JRE for runtime. When depending on a package, take care that you depend on a sufficiently recent version, and explicitly ensure at building time that the providing package gives you the correct interface, i.e. the correct SLOT[USE].


For any package that builds from source, DEPEND should be set to >=virtual/jdk-[minimum-version]:*. If the package does not build from source and is a Java binary, you can use a JRE instead of a JDK in DEPEND.

All java packages will require a JRE for RDEPEND, and should be set to >=virtual/jre-[minimum-version]:*. Unless the package requires a JDK at runtime.

The JDK/JRE atom MUST have a version! Do not use just virtual/jdk or virtual/jre without a version in DEPEND or RDEPEND ever! Otherwise the eclasses won't be able to set minimum -source and -target versions on the package and it would not compile.
CODE Required Java dependencies
CODE Required Java dependencies for optional Java support
DEPEND="java? ( >=virtual/jdk-1.8:* )"
RDEPEND="java? ( >=virtual/jre-1.8:* )"
Never set the JRE version to be less than the JDK version. This causes -target to be less than -source. Which can cause runtime issues, symbols not found, thus should be avoided always! The opposite is safe, though rarely ever required, JRE version greater than JDK version. But that could be used to target newer JRE byte code using older sources.
Never specify the JDK/JRE versions lower than the lowest available JDK/JRE in Gentoo. Lower versions get unsupported by newer JDKs which then breaks compilation and running of such packages with them (for example, JDK 17 does not support versions ≤1.6 which breaks any package depending on JDK ≤1.6).

Some packages do not compile with different JDK versions. If you come across such a package, you have to restrict the compilation to the specific JDK version. Running such a package with newer JDK/JRE is usually safe so you can restrict only the JDK version for compilation.

CODE Package compiling only with JDK 1.8 but running fine with newer JDKs too
Always check whether the package you are working on compiles with newer JDK to prevent breaking of the package when newer JDK version is unmasked for users. This is mostly important for JDK 11 and 17 versions where many changes were introduced. Some issues can be fixed easily (missing dependency etc.) and sometimes it's needed to restrict the JDK version for compilation.

While it is not required, you will commonly see a dependency variable that is used in both DEPEND and RDEPEND. The variable CP_DEPEND is used for anything that will get added to the classpath and go in both DEPEND and RDEPEND. This is very common in most if not all Java ebuilds.

CODE Java dependencies with common ones

If the sources are in a zip file you will need to add app-arch/unzip to BDEPEND (for EAPI 7 and above) (older EAPIs are deprecated).


For Gentoo Java packages, slots are obligatory for every dependency, which also includes packages with SLOT="0". These must end in :0. Failure to adhere to this can cause issues if a package is later slotted. Portage will pull in the latest version, likely the slotted one, removing the un-slotted version, which will break usage of the dependency in the Java ebuild.

Changing Ebuild SLOT

When changing ebuild SLOT, the process of doing it is different for Java packages, because SLOT change affects the installation path of the package and also content of package.env. For that reason, the correct steps to change an ebuild SLOT is to do a revision bump with the new SLOT and then update all the affected packages that should depend on the new SLOT. slotmove (in profiles/updates/) must not be used in this case because it is used only for packages that don't change their instalation content during the SLOT change.


For most Gentoo Java packages you will exclude versions from dependencies, unless you need a specific version of a dependency or a newer version than what might be available. For large Java Applications that are split into multiple packages on Gentoo, it is recommended but not required, to add the version of the package to the dependency, ~category/package-${PV}:${SLOT}. This ensures other packages for the same application will all be built using the same version, in case there could be potential issues mixing pieces of an application with a different version. By having all packages depend on the version and slot, it ensures this cannot happen. Upstreams will likely not address or respond to bugs or issues that arise from using pieces with other pieces from different versions. Nor is it something Gentoo developers or users should be wasting time on.

Take the following example using dev-java/bcmail which depends on dev-java/bcpkix. Both come from the same sources, thus they need to have dev-java/bcmail depend on a specific version of dev-java/bcpkix as follows. This would be the depend for dev-java/bcmail. The version does not matter, nor does the slot. What matters is that we are passing both from the parent package to its dependency, which comes from the same sources.

CODE Example of split package version dependency

Preparing sources

Gentoo Java ebuilds use the normal src_unpack() and src_prepare(). Within src_prepare(), along with doing the normal patching and other things you might need to do in order to prepare the sources, it is policy and practice to always remove any bundled class and/or jar files that came with the package sources, which is one of the first things you should do for any Java package, regardless of optional Java support or not.

Applying patches

CODE Patches will be applied during src_prepare
src_prepare() {
    default #
    java-pkg-2_src_prepare # for any actions (cp, rm, mv, sed or other) except patches

Removing bundled classes and jars

To remove bundled compiled stuff selectively, use java-pkg_clean or JAVA_RM_FILES from java-utils-2.eclass . These can also be used together. The JAVA_PKG_NO_CLEAN array can be used to protect files against java-pkg_clean.

CODE Removing files with java-pkg_clean

src_prepare() {
	# This will remove all .class and .jar files
	# which are not protected by JAVA_PKG_NO_CLEAN.
Starting from EAPI 9 java-pkg_clean will be called automatically.
CODE Removing files with JAVA_RM_FILES
src_prepare() {
    # Good for selective removal in the rare event some bundled
    # binaries/dependencies must be preserved
CODE Removing files with find and JAVA_RM_FILES
src_prepare() {
    for test_file in $(find . -type f -name '*Test*'.java); do

Writing the ebuild using java-ant-2.eclass

See Using_java-ant-2.eclass

Writing the ebuild using java-pkg-simple.eclass

See Using java-pkg-simple.eclass

Typical examples

Using java-pkg-opt-2.eclass

FILE bar-1.0.ebuildExample of a an ebuild with optional Java support
inherit java-pkg-opt-2

DESCRIPTION="Fictional example ebuild"

KEYWORDS="~amd64 ~ia64 ~ppc ~ppc64 ~sparc ~x86"
IUSE="java doc nls"

DEPEND="java? ( >=virtual/jdk-1.8:* )"
RDEPEND="java? ( >=virtual/jre-1.8:* )"

PATCHES=( "${FILESDIR}/${P}.patch" )

src_prepare() {
        default #
        # Remove bundled jars and .class files

src_compile() {
        local myconf="$(use_enable java)"
        if use java; then
                myconf="${myconf} --with-javac-args=\"$(java-pkg_javac-args)\""

        econf $(use_enable nls) ${myconf} || die

        emake || die

src_install() {
        make install DESTDIR=${D} || die

        if use java; then
                java-pkg_newjar build/java/${P}.jar ${PN}.jar

                if use doc; then
                        java-pkg_dohtml -r doc/java


There are some common warnings that are produced by the java compiler. A few are Gentoo specific and others are general. Some are of concern, and others are not.

Bootstrap class path

Anytime the source version is lower than the current java version you will see the following warning.

[options] bootstrap class path not set in conjunction with -source 1.x
This warning was added in java version 1.7 build 121.

While this is a generic warning, it comes from a Gentoo specific issue. Anytime the source version is lower than the current java version, a rt.jar from that older version needs to be set as the bootstrap classpath. This is presently not done on Gentoo, thus the warning.

It is safe to ignore, providing you do not try to run the resulting java binaries on a version of java older than that which it was compiled on. That does somewhat defeat the purpose of using a lower source but sometimes that is necessary for a variety of other reasons.