Parallel Universe

User Guide

Getting Capsule




on Maven Central.

Building Capsule

gradle install


Discuss Capsule on the capsule-user Google Group/Mailing List

Build-Tool Plugins

Using a build-tool plugin may simplify the capsule creation process, especially if you’re doing something clever. You are encourage to use the appropriate community-contributed plugin for you build-tool of choice:

An Example

Before we delve into the specifics of defining a Capsule distribution, let us look at an example of a working capsule. The configurations described below can be built using any JVM build tool. The complete usage example, with both Gradle and Maven build files, is found in the capsule-demo project.

Our capsule JAR has the following structure:

|__ Capsule.class
|__ app.jar
|__ dep1.jar
|__ dep2.jar
|__ agent1.jar

With the manifest (MANIFEST.MF) being:

Manifest-Version: 1.0
Premain-Class: Capsule
Main-Class: Capsule
Application-Version: 1.0
Application-Class: foo.Main
Min-Java-Version: 1.8.0
JVM-Args: -server
System-Properties: my.logging=verbose
Java-Agents: agent1.jar

We embed the application JAR (app.jar), containing the class foo.Main as well as all dependency JARs into the capsule JAR (without extracting them). We also place the Capsule class at the root of JAR. Then, in the JAR’s manifest, we declare Capsule as both the pre-main and main class. This is the class that will be executed when we run java -jar foo.jar. The Application-Class attribute tells Capsule which class to run in the new JVM process, and we set it to the same value, mainClass used by the build’s run task. The Min-Java-Version attribute specifies the JVM version that will be used to run the application. If this version is newer than the Java version used to launch the capsule, Capsule will look for an appropriate JRE installation to use (a maximum version can also be specified). We then list some JVM arguments, system properties and even a Java agent.

When than launch the capsule with java -jar foo.jar. When it runs, it will try to find a JRE 8 (or higher) installation on our machine – whether or not that was the Java version used to launch the capsule – and then start a new JVM process, configured with the given flags and agent, that will run our application.

Launching a Capsule

As mentioned before, java -jar app.jar [app arguments] will launch the application.

The java -Dcapsule.version -jar app.jar command will print the application ID, the version of Capsule used, and then exit without launching the app.

Adding -Dcapsule.log=verbose or -Dcapsule.log=debug before -jar will print information about Capsule’s action.

A Capsule’s Structure and Manifest

A capsule is a JAR file with a manifest – containing meta-data describing the application – and some embedded files. The most minimal (probably useless) capsule contains just the Capsule.class file, and a META-INF/MANIFEST.MF file with the following lines:

Main-Class: Capsule
Premain-Class: Capsule

The existence of Capsule.class in the JAR’s root, as well as those lines in the manifest is what makes a JAR into a capsule (but not yet a launchable one, as we haven’t yet specified an attribute that tells the capsule what to run). The capsule’s manifest describes everything the application requires in order to launch our application. In the next sections we’ll learn the use of various attributes.

Paths and Artifact Coordinates

Many manifest attributes require specifying the location of files. Files can be specified in two ways: as paths, relative to the capsule’s root, or as artifact coordinates. Coordinates take the following form:


(the classifier is optional, as is the version; the minimal coordinates are group:artifact).

The coordinates are the same as those used by Maven repositories to host the artifact. When a file is given as coordinates, capsule will resolve them, by default, by searching for the artifact in the capsule JAR. It will search the following file paths (relative to the capsule’s root):


The type, the artifact’s file extension, is determined by context (i.e. depending on the specific attribute where the coordinates are used), but is usually jar.

The version may be omitted, or may specify a version range, but in those cases, the capsule will just pick any available version, provided there is just one.

Caplets may change the coordinate resolution process. For example, the Maven caplet can resolve coordinates by downloading artifacts from a Maven repository if they are not found in the capsule.

The Application

At the very least, a capsule must have an attribute specifying what application to run. It must be one of:

Application-Class : [the applications's main class, found in the capsule or one of its dependencies]

which specifies the application’s main class, or,

Application : [the coordinates of the application's main executable JAR or the path of the main executable JAR within the capsule]


Application-Script : [a platform specific startup shell/batch-script]

If a script is used, a different one should be listed for each OS type, as we’ll see below.

The Capsule ID

Every capsule has an ID comprised of an application ID and an application version. If the Application attribute is used, and the application is given as artifact coordinates, the application ID will be <group>.<artifact> and the version will be the artifact’s version. Otherwise, the ID and version should be defined with the Application-ID and Application-Version attributes respectively. Application-ID must not contains spaces and should be unique; to ensure uniqueness, it should follow the convention of Java package naming and begin with the author’s or application’s reversed web domain. A good ID is something like

It is also good practice to define the application’s human-readable name, like, “The Foo App”, in the Application-Name attribute.

In most circumstances, the capsule will launch without specifying any of the attributes described in this section, but it is highly recommended that you define them nonetheless.

Capsule’s Cache

By default, Capsule will extract the capsule JAR’s contents – except for class files placed directly in the JAR – into a cache directory. This, however, is an implementation detail and should not be relied upon by applications.

The location of the cache directory is, by default, at ~/.capsule/ on Unix/Linux/Mac OS machines, and at %USERPROFILE%\AppData\Local\capsule\ on Windows. The application caches are placed in the apps/CAPSULE_ID subdirectory of the cache.

If the capsule fails to write to the cache directory (say, the user lacks permission), the cache will be placed in a default, platform specific, temp-file directory, and deleted upon the application’s termination. Otherwise, the capsule will be extracted once. The following run will compare the JAR’s modification date with that of the cache, and will re-write the cache only if the capsule JAR is younger. You can force re-extraction of the capsule with -Dcapsule.reset=true.

The location of the Capsule cache can be changed with environment variables: setting CAPSULE_CACHE_NAME determines the name of the topmost Capsule cache dir (i.e. “capsule” by default), while CAPSULE_CACHE_DIR can be used to set a precise path for the cache (e.g. /tmp/capsule/).

The location of the cache and the location of the JAR are communicated to the application through the capsule.dir and capsule.jar system properties respectively. Capsule defines these properties automatically, and the application may use them, for example, to find extracted resources. In addition, those two filesystem paths can be used within the manifest itself to set various values (system properties, environment variables, etc) by referencing them with $CAPSULE_DIR and $CAPSULE_JAR respectively.

Applications should not generally assume anything about the capsule.dir property defined for them by their capsule. Depending on caplet behavior or even changes to the default capsule implementation, that property may be empty; if it isn’t the directory cannot be assumed to have write permissions.

Modes, Platform- and Version-Specific Configuration

A capsule is configured by attributes in its manifest. Manifest attributes specify which Java version to use when launching the application, what agents to load, the JVM arguments, system properties, dependencies and more.

It is possible to specify different values for each of the configurations – to be selected at launch time – by defining attributes in special manifest sections. Currently, there are three types of sections supported: OS, JRE version and mode. Manifest sections better allow a capsule to adapt to its environment, but are by no means necessary for most capsules.

An OS section is selected based on the OS launching the capsule, and allows defining attributes that are different based on the OS; this is useful for specifying a launch script (in the Application-Script attribute) or native libraries (in the Native-Dependencies/Native-Agents attributes), but can be used for any attribute. The name of the section is the OS name, and can be one of: Windows, MacOS, Linux, Solaris, Unix or POSIX. The Unix section is in effect for any Unix-like system (including Linux and Solaris), and the POSIX section is in effect for Unix-like systems as well as MacOS. So a simple launch script can be defined so:

Name: Windows
Application-Script: run.bat


A JRE version section is selected based on the major Java version chosen by capsule to run the application (as we’ll see later, a capsule can require a specific Java version, or a minimum version). This kind of section is rare, but can be useful for applications that might use different libraries depending on the JRE version used. This kind of section is named Java-N, where N is the major version number (6, 7, 8 etc.).

Modes are a little different, as they are not selected automatically, but by the user when the capsule is launched. Modes are useful in providing different configurations the user can choose from, or even different applications within the same capsule. You define a mode by creating a section that has the mode’s name. For obvious reasons, modes can’t be named in a way that would confuse them with OS or JRE sections, so, for example, you can’t name a mode “Windows”, nor can you name it, Java-9. Here’s an example of the Special mode:

Application-Class: foo.Main

Name: Special
Application-Class: foo.Special

To further specialize modes based on OS or JRE version, simply add - and the OS/JRE name to the modes, like so:

Name: Windows
Application-Script: run.bat


Name: Special-Windows
Application-Script: special.bat

Name: Special-POSIX

A mode is selected by adding -Dcapsule.mode=[mode name] to the capsule’s launch command line. Mode names are case insensitive. A capsule’s modes can be listed by adding -Dcapsule.modes to the command line.

The Application-Name, Application-Id and Application-Version attributes can only be listed in the manifest’s main section; all other attributes may be specialized based on OS, JRE version, or mode. How attribute’s values are combined when different sections are in effect depend on the type of the attribute. If the attribute is a single value (e.g. Application-Class), a section’s attribute overrides the default value (in the main section). If it is a list (e.g. Dependencies), the value is added to the default one. So, for example:

Foo: hi
Bar: 1 2 3
Baz: x y

Name: Linux
Foo: hi-l
Bar: 4

Name: Windows
Foo: hi-w

Name: Special
Bar: 5

Name: Special-Linux
Foo: special-l
Bar: 6

If the mode special is selected, and the capsule is run on a Linux system, the value of the Foo attribute will be special-l, and that of the Bar attribute, because it’s a list attribute, will be: 1 2 3 4 5 6 (because the Linux, Special, and Special-Linux sections are in effect). The Baz attribute is never overridden, and maintains its value of x y.

Selecting the Java Runtime

Two manifest attributes determine which Java installation Capsule will use to launch the application. Min-Java-Version (e.g. 1.7.0_50 or 1.8.0) is the lowest Java version to use, while Java-Version (e.g. 1.6) is the highest major Java version to use. One, both, or neither of these attributes may be specified in the manifest.

First, Capsule will test the current JVM (used to launch the capsule) against Min-Java-Version and Java-Version (if they’re specified). If the version of the current JVM matches the requested range, it will be used to launch the application. If not, Capsule will search for other JVM installations, and use the one with the highest version that matches the requested range. If no matching installation is found, the capsule will fail to launch.

It is also possible to require a minimal update version, say, in cases where the update fixes a bug affecting the application. Because the bug may have been fixed in two different major versions in two different updates (e.g., for Java 7 in update 85, and for Java 8 in update 21), the minimal update required is specified per major Java version, in the Min-Update-Version attribute. For example

Min-Update-Version: 7=85 1.8=21

The major version can be given as a single digit (7) or as a “formal” Java version (1.7, or, 1.7.0). The updates are specified in a whitespace separated list of entries, each entry containing a key (the major version) and a value (the minimum update) as a = separated pair.

If the JDK-Required attribute is set to true, Capsule will only select JDK installations.

Whatever the Min-Java-Version, Java-Version, or JDK-Required attributes specify, launching the capsule with the system property, will use whatever Java installation is specified by the property, for example: java -jar app.jar.

Finally, setting the system property with the path to the executable which will be used to launch the JVM, overrides any JRE selection mechanism.

Running java -Dcapsule.jvms -jar app.jar will list all Java installations Capsule can find, and then quit without launching the app.

Class Paths and Native Libraries

By default, Capsule sets the application’s class path to the capsule JAR itself and every JAR file placed in the capsule’s root – in some unspecified (but constant) order. If the capsule contains an Application attribute, the jar(s) referenced by the attribute will be added to the classpath first.

The classpath, however, can be customized through the App-Class-Path/Dependencies attributes: App-Class-Path should contain a list of file paths that will appear first in the class path, followed by JARs in the capsule root and finally Dependencies should contain artifact coordinates that will be appended to the class path.

For convenience, when specifying the class path, glob-pattern wildcards (*, ?) may be used.

In addition to setting the application classpath, you can also define its boot classpath. The Boot-Class-Path attribute is, similar to the App-Class-Path attribute, an ordered, space separated list of JARs and/or directories relative to the capsule’s root and/or artifact coordinates, that will become the application’s boot classpath. If you don’t want to replace the default Java boot classpath, but simply to tweak it, The Boot-Class-Path-P attribute can be used to specify a classpath to be prepended to the default boot classpath, and the Boot-Class-Path-A attribute can specify a class path that will be appended to the default.

If the capsule is launched with a -Xbootclasspath option, it will override any setting by the capsule’s manifest.

The Library-Path-A manifest attribute can list JARs or directories (relative to the capsule’s root) that will be appended to the application’s native library path. Similarly, Library-Path-P, can be used to prepend JARs or directories to the default native library path.

Native dependencies can be specified in the Native-Dependencies attribute. A native dependency is written as a plain dependency but can be followed by an equal-sign and a new filename to give the artifact (e.g.: com.acme:foo-native-linux-x64:1.0, Each native artifact must be a single native library, with a suffix matching the OS (.so for Linux, .dll for windows, .dylib for Mac). Native libraries too can be embedded in the capsule and are copied into the application cache (and renamed if requested).

JVM Arguments, System Properties, Environment Variables and Agents

The JVM-Args manifest attribute can contain a space-separated list of JVM argument that will be used to launch the application. Any JVM arguments supplied during the capsule’s launch, will override those in the manifest. For example, if the JVM-Args attribute contains -Xmx500m, and the capsule is launched with java -Xmx800m -jar app.jar, then the application will be launched with the -Xmx800m JVM argument.

JVM arguments listed on the command line apply both to the Capsule launch process as well as the application process (see The Capsule Execution Process). Sometimes this is undesirable (e.g. when specifying a debug port, which can only apply to a single process or a port collision will ensue). JVM arguments for at the application process (only) can be supplied by adding -Dcapsule.jvm.args="MY JVM ARGS" to the command line.

The Args manifest attribute can contain a space-separated list of command line arguments to be passed to the application; these will be prepended to any arguments passed to the capsule at launch.

The System-Properties manifest attribute can contain a space-separated list of system properties that will be defined in the launched application. The properties are listed as property=value pairs (or just property for an empty value). Any system properties supplied during the capsule’s launch, will override those in the manifest. For example, if the JVM-Args attribute contains name=Mike, and the capsule is launched with java -Dname=Jason -jar app.jar, then the application will see the name system-property defined as Jason.

The Environment-Variables manifest attribute, is, just like System-Properties, a space-separated list of var=value pairs (or just var for an empty value). The specified values do not overwrite those already defined in the environment, unless they are listed as var:=value rather than var=value.

The Java-Agents attribute can contain a space-separated list with the names of JARs containing Java agents. The agents are listed as agent=params or just agent, where agent is either the path of a JAR embedded in the capsule, relative to the capsule JAR’s root, or the coordinates of a Maven dependency.

Similarly, the Native-Agents attribute may contain a list of native JVMTI agents to be used by the application. The formatting is the same as that for Java-Agents, except that agent is the path to the native library – minus the extension – relative to the capsule root. Unlike in the case of Java-Agents, listing Maven dependencies is not allowed, but the same result can be achieved by listing the dependencies in the Native-Dependencies-... attributes and then referring to the library file’s name in the Native-Agents attribute.

Remember that values listed in all these configuration values can contain the $CAPSULE_DIR and $CAPSULE_JAR variables, discussed in the Capsule’s Cache section.


While Capsule mostly makes startup scripts unnecessary, in some circumstances they can be useful. Capsule allows placing platform-specific scripts into the capsule JAR, and executing them instead of launching a JVM and running Application-Class. The Application-Script attribute specifies the location (relative to the capsule JAR’s root) of the script, and should generally by defined in a platform-specific section.

The scripts can make use of the CAPSULE_DIR environment variable to locate capsule files. In addition, Capsule will choose the JVM installation based on the version requirements, and set the JAVA_HOME environment variable for the script appropriately.

Security Manager

The Security-Policy attribute specifies a Java security policy file to use for the application. Its value is the location of the security file relative to the capsule JAR root. The Security-Policy-A achieves a similar purpose, only the security policy specified is added to the default one rather than replaces it. Finally, the Security-Manager attribute can specify a class to be used as the security manager for the application.

If any of these three properties is set, a security manager will be in effect when the application runs. If the Security-Manager attribute is not set, the default security manager will be used.

Applying Caplets

To apply caplets to the capsule you list them, in the order they’re to be applied, in the Caplets manifest attribute either as class names or as artifact coordinates. If given as artifact coordinates, the caplet JAR file should be placed, unextracted, in the capsule’s root, or, preferably in the capsule directory of the capsule (so as not to be placed on the application’s classpath).

Empty Capsules and Capsule Wrapping

A capsule that contains no application (i.e., it’s manifest has no Application-Class, Application, Application-Name etc.) is known as an empty capsule. Most caplets and, indeed, the Capsule project itself, are shipped as binaries which are essentially empty capsules. While you cannot run an empty capsule on its own, empty capsules can serve – unmodified – as capsule wrappers that wrap other capsules, or even un-capsuled applications. This is most useful when the empty, wrapper, capsule employs caplets to provide some special behavior to the wrapped capsule or application.

Here’s how we use an empty capsule to launch an application stored in a local JAR:

java -jar capsule.jar coolapp.jar

In both cases, the only requirement from coolapp is that it has a main class declared in its manifest. If coolapp is a capsule rather than a simple application, then our empty capsule’s caplets will be applied to the wrapped capsule.

“Really Executable” Capsules

A JAR file can be made “really executable” in UNIX/Linux/MacOS environments – i.e. it can be run simply as capsule.jar ARGS rather than java -jar capsule.jar ARGS – by prepending a couple of shell script lines to the JAR (it turns out JAR files can tolerate any prepended headers).

Both capsule-maven-plugin and gradle-capsule-plugin support creation of really executable capsules simply by setting a flag.

If you choose not to use one of the capsule plugins, then you can use the really-executable-jars Maven plugin to make your capsule really executable (or if you’re using the capsule-maven-plugin, just set the buildExec tag to true). In Gradle, this can be done by adding the following function to your build file:

def reallyExecutable(jar) {
    ant.concat(destfile: "tmp.jar", binary: true) {
        zipentry(zipfile: configurations.capsule.singleFile, name: 'capsule/')
        fileset(dir: jar.destinationDir) {
            include(name: jar.archiveName)
    copy {
        from 'tmp.jar'
        into jar.destinationDir
        rename { jar.archiveName }
    delete 'tmp.jar'

and then

capsule.doLast { task -> reallyExecutable(task) }

The Capsule Execution Process

When a capsule is launched, two processes are involved: first, a JVM process runs the capsule launcher, which then starts up a second, child process that runs the actual application. The two processes are linked so that killing or suspending one, will do the same for the other. On UNIX systems (Linux, Mac, etc.), the launcher process makes public the identity of the child process by setting the system property, which can be queried with the jcmd command. Suppose the capsule’s pid is 1234, then the child (app) process pid can be obtained with the following shell command:

jcmd 1234 VM.system_properties  | grep | cut -f 2 -d =

While this model works well enough in most scenarios, sometimes it is desirable to directly launch the process running the application, rather than indirectly. This is supported by “capsule trampoline”, and is available for really executable capsules on UNIX systems only. To take advantage of capsule trampolining, use the capsule/ executable header (rather than capsule/ when creating the really executable capsule.

Writing Caplets

You can customize many of the capsule’s inner workings by creating a caplet – custom capsule. A caplet is a subclass of the Capsule class that overrides some of its overridable methods.

Please consult Capsule’s Javadoc for specific documentation on custom capsules.