Modules API - NetBeans API Javadoc (Current Development Version)

Overview

Javadoc

Contents

• What Are Modules?
  • Related standards
• How to Create a New Module
  • Build a JAR
  • Writing the manifest
  • Using a custom module class
    • "Installation-clean" modules
  • Using an installation layer
  • Using versioning
  • Writing an OS dependent module
  • Localization and branding support
• Standard Module Sections
  • Action
  • Data loader
• Deployment of Modules
  • Related Files
  • Deprecation
  • Refactoring
  • Downloading
  • Module install order
  • List of all modules
  • Security
  • JNI

Modules API

How to Create a New Module

For basic cases, you should need to do very little to create a module besides writing the basic source code.

Build a JAR

All module implementation classes must reside in a JAR file. If you want to split up a large application into several pieces, perhaps so as to make independent upgrades possible, you should do so by creating multiple modules and relating them using versioning.

Writing the manifest

There are a few other global tags which are optional but encouraged:

OpenIDE-Module-Name

Gives a human-presentable display name for the module. The name may be localized by adding additional tags for each locale, e.g. OpenIDE-Module-Name_fr.

OpenIDE-Module-Short-Description

A short description of what the module does (about one sentence, like a tool tip). May be localized as for the display name.

OpenIDE-Module-Long-Description

A longer description of what the module does (roughly paragraph-length). May be localized as for the display name.

OpenIDE-Module-Display-Category

A phrase giving a category for the module. Modules with the same category may be visually grouped together in various parts of the UI. May be localized as for the display name.

OpenIDE-Module-Install

The name of a custom module class.

OpenIDE-Module-Layer

The name of an installation layer.

Various versioning- and dependency-specific tags

Please see the section on versioning for details on these.

OpenIDE-Module-Module-Dependency-Message and OpenIDE-Module-Package-Dependency-Message

Localizable pleasant messages to display to a user in case a module or package dependency fails. In some cases it may be quite normal for a dependency to fail and it is desirable to provide a specific and helpful message to the user explaining where to get the required dependency or why the module depending on it is not needed by this user. May be localized as for the display name. Since 1.26

OpenIDE-Module-Requires-Message

As above, localizable pleasant message to display when a required token is not found. Since 2.3

Since 1.24: A more flexible way to provide human-readable information is to declare the attribute OpenIDE-Module-Localizing-Bundle. Its value should be the resource path to the (base locale of) a bundle file providing localizable attributes for the module (display name, category, short and long description). The bundle file may have localized variants as usual, and the keys should be the same as the desired manifest attribute names, e.g. OpenIDE-Module-Name. If it is necessary to localize an attribute from a section (currently only filesystem sections with their Display-Name attribute), you may again place these in the bundle, where the key will be prefixed by the section name and a slash, e.g. org/foo/MyFileSystem.class/Display-Name.

All other tags are bound to a particular module section. Naturally you may use other standard manifest attributes.

Manifest-Version: 1.0
OpenIDE-Module: com.modulemakers.clip_disp/2
OpenIDE-Module-Specification-Version: 2.0.1
OpenIDE-Module-Implementation-Version: 2.0-beta-rewrite
OpenIDE-Module-Build-Version: 2003-12-31 00:23 cron@buildhost.mycorp.com
OpenIDE-Module-Name: Clipboard Displayer
OpenIDE-Module-Name_cs: Prohlizec schranky
OpenIDE-Module-Install: com/modulemakers/clip_disp/Installer.class
OpenIDE-Module-Layer: com/modulemakers/clip_disp/Layer.xml
X-Comment-1: I am a comment (just a deliberately meaningless
X-Comment-2: header) - "Sealed" is a standard manifest attribute.
Sealed: true

Name: com/modulemakers/clip_disp/DisplayClipboardAction.class
OpenIDE-Module-Class: Action

Using a custom module class

To use a main install class, just extend the ModuleInstall base class. Your class must be able to be instantiated as a JavaBean. There are several methods which you may override, and may do anything which is required to make the module cleanly enter and exit NetBeans.

package com.modulemakers.clip_disp;
import org.openide.modules.ModuleInstall;
import org.openide.filesystems.FileUtil;
import java.net.*;

public class ModuleHandler extends ModuleInstall {
  public void installed() {
    // This module has been installed for the first time! Notify authors.
    HttpURLConnection conn = (HttpURLConnection)
      (new URL ("http://www.modulemakers.com/clip_disp/installed.cgi").openConnection ());
    conn.getResponseCode ();
    conn.disconnect ();
    // Handle setup within this session too:
    restored ();
  }

  // Nothing special required here.
  // public void restored() {
  // }

  // Do not need to do anything special on uninstall.
  // Tools action will be removed automatically.
  // public void uninstalled() {
  // }

  public boolean closing() {
    // Ask the user to save any open, modified clipboard contents.
    // If the user selects "Cancel" on one of these dialogs, don't exit yet!
    return DisplayClipboardAction.askAboutExiting ();
  }
}

The installed handler may do more or less what it wishes - it will be called in a running NetBeans instance. The same applies to uninstalled and closing. However, restored is more delicate in that it may be called during earlier phases of NetBeans initialization. Therefore, it should not use presentation-oriented features of NetBeans, such as the Explorer or Window System APIs. However, the other APIs are acceptable to use in restored, including DialogDisplayer.

The ModuleInstall.updated(...) method will be called just once when a module is updated to a new version - when the new version is loaded into NetBeans for the first time (in a new session), the method is called and the previous version number is accessible. Neither installed nor uninstalled will be (automatically) called in this case. It is a module author's responsibility to make sure that new versions of a module are capable of "cleaning up" obsoleted installations created by older versions, as far back as a user is likely to be directly upgrading.

Some module installer classes may desire to keep state across sessions, for example if they require specific instructions on what to uninstall that can only be gotten during the installation process. All installers are automatically Externalizable and module authors may decide to override the two methods of this interface to read and write its state from an object source. Typically you would want to serialize and deserialize shared class fields in the object using these methods. It is a good idea to call the super methods as the first action when overriding.

In some cases, a module relies on some external resource to be present in order for it to be used. For example, an external application may need to be installed in order for it to do anything. Or it may require a valid license key. In such cases, the ModuleInstall.validate() method may be overridden to check for this external resource and throw a (preferably politely localized) IllegalStateException if something is wrong. Throwing this exception will cancel the module installation (or restoration) cleanly. Note that such an installer should not expect anything related to the module to already be loaded when this method is called, as it may be done when deciding whether to even begin loading.

Using an installation layer

You may specify the global tag OpenIDE-Module-Layer in addition to or instead of a module main class; as a rule, use a layer for a particular task in preference to a module installer, if you can. The tag value should point to an XML file inside the module which is in the format understood by XMLFileSystem. Its contents specify some files that the module will add to the system filesystem controlling much of NetBeans' configuration. This filesystem is composed of many read-only XML layer filesystems as well as a writable layer corresponding to the config subdirectory of the user directory.

The XML format is relatively simple. There is an online DTD for it, but descriptively: the document element is <filesystem> and corresponds to the root folder; subfolders are represented with the <folder> element; and files within folders with the <file> element. Files and folders must have names, with the name attribute. Files need not specify contents, in which case they will be zero-length; or they may specify contents loaded from some other resource (url attribute, treated as relative to the base URL of the document itself); or for small textual contents, the contents may be included inline inside the element, typically using the CDATA syntax, though this usage is deprecated. Attributes may be specified on folders or files as empty <attr> elements; the name must be supplied, and the value in one of several formats: primitive types, strings, URLs, arbitrary serialized objects in hexadecimal, or computed on the fly by calling the default constructor of some class, or a static method (the method may be passed the file object in question and/or the attribute name, if you wish).

<?xml version="1.0"?>
<!DOCTYPE filesystem PUBLIC
    "-//NetBeans//DTD Filesystem 1.0//EN"
    "http://www.netbeans.org/dtds/filesystem-1_0.dtd">
<filesystem>
    <folder name="Templates">
        <folder name="Other">
            <file name="foo.txt" url="actual-contents.txt">
                <!-- Make it a template: -->
                <attr name="template" boolvalue="true"/>
                <!-- Localized display name of data node: -->
                <attr name="SystemFileSystem.localizingBundle"
                      stringvalue="com.foo.module.Bundle"/>
                <!-- HTML description of template for wizard to display: -->
                <attr name="templateWizardURL"
                      urlvalue="nbresloc:/com/foo/module/foo-template-help.html"/>
                <!-- Set an iterator; complex object, so -->
                <!-- easiest to create thus: -->
                <attr name="templateWizardIterator"
                      newvalue="com.foo.module.FooTemplateIterator"/>
            </file>
        </folder>
    </folder>
</filesystem>

By default files listed in XML layers are not ordered in any particular way, just as files found on disk would not have a particular order. For purposes of some kinds of installation, it is desirable to order data objects in data folders in a particular way - for example, menus are built from their menu items in the order the instance-bearing data objects occur in the data folder. The call DataFolder.setOrder(DataObject[]) suffices to arbitrarily change folder order, but usually it is desirable to create the proper order declaratively in the XML. For this reason, DataFolder understands special attributes (set on the FileObject forming the folder) which provide relative ordering constraints on the objects in the folder. Specifically: if there are data objects A and B in the folder, represented by primary files afile and bfile, and there is an attribute on the folder containing them named afile/bfile whose value is Boolean.TRUE, then the folder will attempt to place A before B in its ordering. For example, the XML:

<folder name="SomeFolder">
    <file name="first.txt" url="first.txt"/>
    <attr name="first.txt/second.txt" boolvalue="true"/>
    <file name="second.txt" url="second.txt"/>
</folder>

1. Modules may provide layers some of whose files specify relative constraints on files in the same folder position but installed by the core, or other modules. That is fine and the constraints will be honored - assuming the files being constrained both exist. Constraints for which one or both of the affected files do not exist, are simply ignored.
2. It is fine to provide multiple constraints from a single file, for example if there are several possible files it might need to be installed after; all relevant constraints will be attempted.
3. If constraints are contradictory, they may be ignored (and a warning issued).
4. If the user reorders the folder manually or DataFolder.setOrder(DataObject[]) is called, the explicit order overrides the existing constraints. Subsequently added constraints might have an effect, however.
5. Only the names of primary files from data objects have any bearing on the ordering.
6. A constraint specifying that A comes before B does not force the folder to place it immediately before B. If you want to ensure that A in fact comes after some other files you know come before B, thus trying to make them adjacent, this must be done explicitly. Another module might explicitly interpose a file C between A and B.

The resource path pointing to the layer is automatically localized by NetBeans every time the module is installed or restored; if there are locale-specific variants, they are merged with the main layer. More-specific variants can simply add files to the set installed by the module; however they take precedence over the less-specific variants, and thus can replace files, or even remove (suppress) them, using *_hidden masks as used by MultiFileSystem. In fact, if module A depends on module B, then A's layer(s) will take precedence over B's layers, and it may replace or remove files installed by B. In the same way, any module may replace or remove files installed by the core.

Reverting user's modifications

When a user makes any changes in the installation layer - usually through the user interface by changing the default values for some application options or any other customizations - it may be desirable to revert these changes back to their defaults as defined in XML layers. There changes are stored in user's working dir.

Since version 7.2 the FileObject supports a new attribute "removeWritable" returning an instance of Callable which removes the local writable version of the given FileObject thus reverting the folder or file to its initial state as defined in XML layers. Please note that is *not* possible to reset FileObject's attributes.

Using versioning

Modules can do three things with versioning:

1. Specify what they are. This is done by the special OpenIDE-Module tag, whose value should be a unique (programmatic) identifier for the module, as mentioned above. Not be confused with the display name, which is free-form and may be changed at will, this code name should not be changed arbitrarily - pick a name and stick with it throughout module releases.
2. Specify which version they are. In line with the Java Versioning Specification, modules can indicate two pieces of version information about themselves using the OpenIDE-Module-Specification-Version and the OpenIDE-Module-Implementation-Version tags. Modules are also permitted to use OpenIDE-Module-Build-Version to give information about when or by whom they were physically built, in case there is more specialized semantics given to the implementation version.
3. Specify which features they depend on. Again, this is done using the Versioning Specification - modules can request general or specific versions of other modules (OpenIDE-Module-Module-Dependencies), Java packages (OpenIDE-Module-Package-Dependencies), or Java itself (OpenIDE-Module-Java-Dependencies).

   The tags OpenIDE-Module-Provides and OpenIDE-Module-Requires can also be used to specify dependencies between modules without naming the exact module to depend on. A module may provide one or more tokens. These are strings of conventional meaning, in the format of a Java package or class name - perhaps taken to be the name of a class which will be supplied to the lookup system, though there could be other meanings. A module may also require one or more tokens. A module which requires some tokens may only be enabled by the system if for each such token, there is at least one other module which provides that token and is already enabled. Since version 7.1 there is also support for OpenIDE-Module-Needs which is a weaker version of requires as it does not impose any restriction on the ordering of module. The OpenIDE-Module-Needs is useful for modules that define some API and require an implementation of it. Just specify the need for an implementation and make other modules depend on your module and OpenIDE-Module-Provides the implementation token. Moreover there is also OpenIDE-Module-Recommends which is even weaker version as it creates a conditional dependency - e.g. enables the module providing the token only if it is available, however if it is not, no dependency is broken.

(You may also specify a build version in your manifest, typically giving a date. This is used only for diagnostic purposes and is otherwise ignored by the module infrastructure.)

So, modules may specify either or both kinds of versions in their manifest, and correspondingly request such versions from other modules. (You may have at most one dependency between any single pair of modules, however.) The Versioning Specification provides similar numbering for (some, but not all) Java packages, as well as the core Java Platform APIs and Virtual Machine Specification. NetBeans also has such versions which may be used to request a general level of Open API support (using specification versions) or a particular NetBeans build (using implementation versions).

Since the Versioning Specification does not address incompatible changes (those which would break existing code), but these are in practice occasionally necessary (however painful), modules may provide a separate integral number giving the incompatible release version. You may only request a particular release version, not subsequent ones, and you must specify the release version if there is one when giving any module dependency. It only differs from a complete change of the feature in that the module installer tool may prompt the user to make a "major upgrade" if a new release version of a module is available.

Since 2.3 a module dependency may also use a range of release versions. The syntax is that in place of a single major release version, you give a minimum then maximum version separated by dashes:

OpenIDE-Module-Module-Dependencies: base.module/1-2 > 1.0

where the base module might either have the minimum major release version (and the given specification version or later, if that is also requested), or other major release versions up to the maximum (in which case the specification version is irrelevant). You may not ask for an implementation version with this syntax.

To use all of these tags once you understand them is not too hard. First of all, feature names: a module is named according to the OpenIDE-Module tag, which should look like a Java package name, but may be followed by a slash and release number (e.g. com.mycom.mymodule/3); Java packages are just named by the package name; the Java Platform APIs are named by Java; and the Java Virtual Machine by VM.

Now, a module may list its own specification and/or implementation versions using the tags above. To specify a dependency on other features, it may use some or all of the four dependency tags. The value of each will be a comma-separated list of dependencies of that general type, each of which can be of the form:

FEATURENAME

Just requests that the feature be present, not any particular version. Useful for modules and Java packages (usually, Java standard extensions) that need to be installed, but that is all. This style is also used for requiring tokens, which cannot have versions.

FEATURENAME > SPECVERSION

Requests that the feature be present, that it state a specification version, and that this version be greater than or equal to the requested version. Note that you may not request an exact specification version. This is the preferred form for module dependencies.

Remember, if requesting a dependency on a module which has a release number, you must give that release number as part of the feature name, whether or not you also ask for a specification version.

FEATURENAME = IMPLVERSION

Requests that the feature be present, that it state an implementation version, and that this version exactly match the requested one. Note that you may not request a comparison on implementation versions, as they are generally non-numeric and not comparable.

OpenIDE-Module: com.mycom.mymodule/1
OpenIDE-Module-Specification-Version: 1.0.1
OpenIDE-Module-Implementation-Version: 1.0-release-g
OpenIDE-Module-Module-Dependencies:
 com.mycom.mysistermodule/1 = 1.0-release-g,
 com.othercom.anothermodule > 2.1,
 org.netbeans.modules.applet/1 > 1.0
OpenIDE-Module-Package-Dependencies: javax.television > 0.9
OpenIDE-Module-Java-Dependencies: Java = 1.2.1b4, VM > 1.0
OpenIDE-Module-Provides: javax.television.TunerProvider, javax.television.RemoteControl
OpenIDE-Module-Requires: org.netbeans.javahelp.api.Help

There is further special treatment for handling of package dependencies for several reasons:

1. In practice, the Java Versioning Specification is not all that widely followed, and many needed extensions will not list this information in their manifest.
2. Class loaders only define packages when the first class from that package is actually loaded. This makes it more difficult to verify whether a package is really there or not.
3. The standard manifest attribute Class-Path must be recognized and used; typically this attribute is used to actually add extensions to the module classloader, while the Modules API attributes are used to ensure that specific versions are available.

You may specify a sample class name in square brackets immediately after the package name (or indeed instead of it). Giving a class name together with a package name requests that NetBeans first try to load that class (it must be able to, so choose a class in the extension you know will be there); then the versioning information is checked. (If the sample class is simply in the desired package, you may omit the package qualification as a shortcut.) This addresses the second problem. Giving just a class name in square brackets with no preceding package name indicates that NetBeans should only ensure that the named class is loadable, bypassing the Versioning specification mechanisms. This addresses the first problem.

OpenIDE-Module: com.mycom.mymodule/1
OpenIDE-Module-Package-Dependencies: javax.television[TunerProvider] > 0.9
Class-Path: ext/television.jar

Important! If your module has compile-time references to classes in another module, you must specify a direct dependency on that module using OpenIDE-Module-Module-Dependencies. This ensures that the modules will be installed in the correct order; otherwise a NoClassDefFoundError or similar problem could result.

Since 2.19 modules which provide Java-level APIs can specify that only certain packages in the module form part of the public API and should be accessible to other modules depending on that module. The tag OpenIDE-Module-Public-Packages is optional; if not present, all packages are considered fair game. If present, it gives a list of package names to export to other modules, for example:

OpenIDE-Module-Public-Packages: org.netbeans.api.foo.*

indicates that only classes (and resources) in the package org.netbeans.api.foo should be available to client modules. Attempts to use classes from other packages will fail, for example with a NoClassDefFoundError.

More than one package can be given; separate them with commas or spaces. A package name may end in .** rather than .* to indicate that any subpackages are also exported, rather than just the package itself. The special value - (a single dash) indicates that no packages are to be exported. (The module might still provide a non-Java-level API, for example by defining and handling a custom XML DTD.) "Private" classes and resources may still be accessed using reflection from the system classloader, just not from a module classloader.

Additionally, sometimes a module with an API in public packages wishes to provide access only to certain "friend" modules. This is possible if the module declares public packages together with a list of code base names of "friend" modules that can access them:

OpenIDE-Module-Friends: org.foo, org.boo

Additionally, sometimes a module wishes to get unrestricted access to non-public packages of an API module. This is discouraged, but possible if such module declares a direct dependency on the API module using an implementation version dependency - this kind of dependency indicates that the client module is prepared to track every idiosyncrasy of the API module, and knows how to safely use undocumented classes, as if both modules formed part of the same code base.

Confused by all these restrictions on classloading? See the Class Path document which gives a fuller, more informal explanation.

Writing an OS dependent module

The tokens supported in version 4.44 are:

OpenIDE-Module-Requires: org.openide.modules.os.Windows
OpenIDE-Module-Requires: org.openide.modules.os.Unix
OpenIDE-Module-Requires: org.openide.modules.os.MacOSX

OpenIDE-Module-Requires: org.openide.modules.os.OS2
OpenIDE-Module-Requires: org.openide.modules.os.PlainUnix    

Please note, that Mac OS X is in fact also Unix, so requesting Unix also enables your module on Mac OS X. If you want all unix types but Mac OS X you want to request PlainUnix token.

So if one wants a module to be automatically enabled on Mac OS X and silently disabled on other platforms the best way is to create an eager module which requests the MacOSX token and the module system takes care of enabling it only on Mac OS X (as is the case of NetBeans applemenu module).

Localization and branding support

Existing Internationalization, Localization, and Branding documentation describes how IDE and platform is localized and general principles are valid for all modules.

Standard Module Sections

Action

By using this manifest section, your action will be available as a "tool", i.e. it will appear (when enabled) in a list of all extension tools in any popup menu (etc.) using ToolsAction. Note that you do not need to use ToolsAction directly for this purpose: it should already be attached as a popup to all interesting nodes, as a menu item in the Tools menu, etc. Just using the manifest tag ensures that your action will be shown under such submenus.

Loader

There are a couple of options which may be used to specify the relative precedence of data loaders, so that a more specific loader may take precedence over a more general one:

Install-Before: classname

Instructs NetBeans to try to install this data loader before another, so as to give it higher precedence: it will be given the opportunity to scan over potential data objects first. The classname should specify the representation class of the other loader's data object type; see the DataLoader constructor for an explanation of how to use representation classes. You may supply multiple class names separated by commas.

Install-After: classname

Exactly like Install-Before, except that NetBeans will attempt to install this loader after another, so as to give it lower precedence.

You need not specify module dependencies on other modules simply because you use the names of data objects in those modules in your install before and after tags; the ordering hints will simply be ignored unless and until the other module is installed, at which time they will take effect.

Deployment of Modules

This section describes how a module may be deployed to a user's NetBeans instance.

Related Files

Sometimes a module may need to bundle other files besides the module JAR alongside it. No concrete mechanism for doing so is specified in the Modules API. However, if there is such a mechanism, then it is possible for the module to find these other files. InstalledFileLocator permits such packaging mechanisms to let modules find their associated files, as of 3.21.

In the current implementation, modules may be bundled with other files using NBM files as provided by Auto Update, and the NetBeans build process premerges modules in the selected module config into one large installation directory, accessible via several system properties. Correspondingly, the default locator finds files in this structure. The installation structure might however change in the future, so InstalledFileLocator is the only safe way to find such bundled resources.

Deprecation

Sometimes you may have an API module which you wish to deprecate in its entirely - it is only available for purposes of backwards compatibility. Normally such modules are autoloads. To ensure that remaining clients of the module are properly warned of its status, you may specify the manifest attribute OpenIDE-Module-Deprecated with the value true. Generally you should also supply a message using the localized manifest attribute OpenIDE-Module-Deprecation-Message. This message may be displayed somehow if a non-deprecated module depending on your deprecated module is enabled.

Refactoring

Sometimes as the developer of an API module, you may wish to make major changes in how your APIs are laid out physically in modules. Most commonly, you determine that it would be desirable to split one big API module into several smaller pieces. However you wish to retain compatibility for existing client modules, so that if they used your old monolithic module, they will now automatically get access to the newer, smaller pieces.

This is straightforward: create an XML file according to the DTD -//NetBeans//DTD Module Automatic Dependencies 1.0//EN and install it in the system filesystem under ModuleAutoDeps/ (system/ModuleAutoDeps/*.xml). Note that it will probably not work to do this using an XML layer since the information must be available before any module is even loaded.

As of 3.33

Module install order

First of all, when NetBeans starts up, all previously-installed modules are restored, calling the restored methods on their install classes if present. (Sections are installed before this method is called.) If multiple modules are being restored (which is normally the case), NetBeans installs them in an arbitrary order. However, if some of them specify dependencies on other modules, the order may be adjusted so as to make sure that the one specifying the dependency is installed after the one it depends on.

Now, when NetBeans is running, modules may be installed into it, possibly a cluster of them at once. In such a case, NetBeans first checks to make sure that all the prospective modules satisfy their dependencies, either on system or Java features; already-installed modules; or other modules in the prospective cluster. If any modules fail their dependencies (or had syntactical errors in their manifest files, etc.), they are removed from the list and the user is given an explanation of what is missing. The remainder are then ordered according to dependencies as above, and then installed in that order.

Caution: it is forbidden to install modules which have cyclic dependencies on one another, as NetBeans would be unable to determine which order to install them in! In fact, it will signal an error and not install such modules. Generally such a situation means that you should "factor out" the common required functionality into a new module which the original ones will then specify legal dependencies on.

List of all modules

Collection<ModuleInfo> modules = Lookup.getDefault().lookupAll(ModuleInfo.class);

A limited ability to modify the installation status of modules is provided by the APIs as of version 1.31. In the folder Modules/ on the system filesystem, there will be XML files corresponding to all known modules. In each case the name of the XML file is derived from the code name base of the module with . replaced by -, and followed by .xml; for example, org-netbeans-modules-properties.xml. The contents of the XML file are given by a fixed DTD.

For purposes of the API, only parts of these files are defined. The root element must be <module> and its name attribute must be the code name base of the module. Inside the root element are various <param> elements, each with a name and some textual contents. The APIs define only one parameter, enabled, which if present must be either true or false.

The APIs do not currently permit arbitrary changes to these files. Specifically, addition or deletion of these files, or modification of other files that may be in the same folder, is not permitted. Modification of these files is permitted in only one way: if a module XML file already contains the parameter enabled, you may rewrite the file to be identical except with the opposite value of this parameter. The module system will then make a best effort to enable or disable the module accordingly (subject to dependencies and other constraints).

Doing this is not recommended except in unusual circumstances, and is intended primarily for session support to be able to enable and disable modules via layer. If your module just needs to prevent itself from being turned on under some circumstances (for example if it is missing a valid license key), simply use ModuleInstall.validate.

When rewriting the module XML be sure to use FileSystem.runAtomic(FileSystem.AtomicAction) to wrap the reading of the old XML and the writing of the new, to prevent file changes from being fired halfway through.

JNI

It is possible to run modules making use of JNI native implementations inside NetBeans. You may create a directory modules/lib/ (i.e. a subdirectory lib/ beneath the directory where the module JAR resides) and place native libraries (DLL or shared-object) there; it will be added to the search path for calls to System.loadLibrary made from module code. This avoids the need to explicitly add some directory to the binary load path. The module is entirely responsible for distinguishing between various platforms and operating systems and requesting a library name appropriate to the current one, however. Use of JNI is of course not recommended and should be restricted to cases where it is unavoidable.

Example of usage: if on Linux a module located in /home/app/cluster/modules/something.jar calls System.loadLibrary("stuff-linux-i386"), the library should be in /home/app/cluster/modules/lib/libstuff-linux-i386.so. On Windows, if C:\app\cluster\modules\something.jar calls System.loadLibrary("stuff"), the library should be in C:\app\cluster\modules\lib\stuff.dll. (Remember that Java has platform-specific prefixes and suffixes it adds to plain library names before trying to find it on disk.)

If a native library refers to other native libraries, they are likely to be found using the normal search path for the platform. This may mean that if you have several libraries used in a module, you must either put all but the directly referenced one in the system's global library search path, or merge them all together.

Warning: since the JVM cannot load the same native library twice even in different classloaders, a module making use of this feature cannot be enabled more than once in a single VM session. JARs loaded from the classpath (application classloader) are never reloaded, so it may be possible to include the native-dependent classes in such a JAR and make use of it from a module JAR.