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3.6. Use the Class Loader Programmatically in a Deployment

3.6.1. Programmatically Load Classes and Resources in a Deployment

You can programmatically find or load classes and resources in your application code. The method you choose will depend on a number of factors. This topic describes the methods available and provides guidelines for when to use them.
Load a Class Using the Class.forName() Method
You can use the Class.forName() method to programmatically load and initialize classes. This method has two signatures.
Class.forName(String className)
This signature takes only one parameter, the name of the class you need to load. With this method signature, the class is loaded by the class loader of the current class and initializes the newly loaded class by default.
Class.forName(String className, boolean initialize, ClassLoader loader)
This signature expects three parameters: the class name, a boolean value that specifies whether to initialize the class, and the ClassLoader that should load the class.
The three argument signature is the recommended way to programmatically load a class. This signature allows you to control whether you want the target class to be initialized upon load. It is also more efficient to obtain and provide the class loader because the JVM does not need to examine the call stack to determine which class loader to use. Assuming the class containing the code is named CurrentClass, you can obtain the class's class loader using CurrentClass.class.getClassLoader() method.
The following example provides the class loader to load and initialize the TargetClass class:

Example 3.7. Provide a class loader to load and initialize the TargetClass.

Class<?> targetClass = Class.forName("com.myorg.util.TargetClass", true, CurrentClass.class.getClassLoader());
Find All Resources with a Given Name
If you know the name and path of a resource, the best way to load it directly is to use the standard Java development kit Class or ClassLoader API.
Load a Single Resource
To load a single resource located in the same directory as your class or another class in your deployment, you can use the Class.getResourceAsStream() method.

Example 3.8. Load a single resource in your deployment.

InputStream inputStream = CurrentClass.class.getResourceAsStream("targetResourceName");
Load All Instances of a Single Resource
To load all instances of a single resource that are visible to your deployment's class loader, use the Class.getClassLoader().getResources(String resourceName) method, where resourceName is the fully qualified path of the resource. This method returns an Enumeration of all URL objects for resources accessible by the class loader with the given name. You can then iterate through the array of URLs to open each stream using the openStream() method.

Example 3.9. Load all instances of a resource and iterate through the result.

Enumeration<URL> urls = CurrentClass.class.getClassLoader().getResources("full/path/to/resource");
while (urls.hasMoreElements()) {
    URL url = urls.nextElement();
    InputStream inputStream = null;
    try {
        inputStream = url.openStream();
        // Process the inputStream
        ...
    } catch(IOException ioException) {
        // Handle the error
    } finally {
        if (inputStream != null) {
            try {
                inputStream.close();
            } catch (Exception e) {
                // ignore
            }
        }
    }
}

Note

Because the URL instances are loaded from local storage, it is not necessary to use the openConnection() or other related methods. Streams are much simpler to use and minimize the complexity of the code.
Load a Class File From the Class Loader
If a class has already been loaded, you can load the class file that corresponds to that class using the following syntax:

Example 3.10. Load a class file for a class that has been loaded.

InputStream inputStream = CurrentClass.class.getResourceAsStream(TargetClass.class.getSimpleName() + ".class");
If the class is not yet loaded, you must use the class loader and translate the path:

Example 3.11. Load a class file for a class that has not been loaded.

String className = "com.myorg.util.TargetClass"
InputStream inputStream = CurrentClass.class.getClassLoader().getResourceAsStream(className.replace('.', '/') + ".class");

3.6.2. Programmatically Iterate Resources in a Deployment

The JBoss Modules library provides several APIs for iterating all deployment resources. The JavaDoc for the JBoss Modules API is located here: http://docs.jboss.org/jbossmodules/1.3.0.Final/api/. To use these APIs, you must add the following dependency to the MANIFEST.MF:
Dependencies: org.jboss.modules
It is important to note that while these APIs provide increased flexibility, they will also run much more slowly than a direct path lookup.
This topic describes some of the ways you can programmatically iterate through resources in your application code.
List Resources Within a Deployment and Within All Imports
There are times when it is not possible to look up resources by the exact path. For example, the exact path may not be known or you may need to examine more than one file in a given path. In this case, the JBoss Modules library provides several APIs for iterating all deployment resources. You can iterate through resources in a deployment by utilizing one of two methods.
Iterate All Resources Found in a Single Module
The ModuleClassLoader.iterateResources() method iterates all the resources within this module class loader. This method takes two arguments: the starting directory name to search and a boolean that specifies whether it should recurse into subdirectories.
The following example demonstrates how to obtain the ModuleClassLoader and obtain the iterator for resources in the bin/ directory, recursing into subdirectories.

Example 3.12. Find resources in the "bin" directory, recursing into subdirectories.

ModuleClassLoader moduleClassLoader = (ModuleClassLoader) TargetClass.class.getClassLoader();
Iterator<Resource> mclResources = moduleClassLoader.iterateResources("bin",true);
The resultant iterator may be used to examine each matching resource and query its name and size (if available), open a readable stream, or acquire a URL for the resource.
Iterate All Resources Found in a Single Module and Imported Resources
The Module.iterateResources() method iterates all the resources within this module class loader, including the resources that are imported into the module. This method returns a much larger set than the previous method. This method requires an argument, which is a filter that narrows the result to a specific pattern. Alternatively, PathFilters.acceptAll() can be supplied to return the entire set.

Example 3.13. Find the entire set of resources in this module, including imports.

ModuleClassLoader moduleClassLoader = (ModuleClassLoader) TargetClass.class.getClassLoader();
Module module = moduleClassLoader.getModule();
Iterator<Resource> moduleResources = module.iterateResources(PathFilters.acceptAll());
Find All Resources That Match a Pattern
If you need to find only specific resources within your deployment or within your deployment's full import set, you need to filter the resource iteration. The JBoss Modules filtering APIs give you several tools to accomplish this.
Examine the Full Set of Dependencies
If you need to examine the full set of dependencies, you can use the Module.iterateResources() method's PathFilter parameter to check the name of each resource for a match.
Examine Deployment Dependencies
If you need to look only within the deployment, use the ModuleClassLoader.iterateResources() method. However, you must use additional methods to filter the resultant iterator. The PathFilters.filtered() method can provide a filtered view of a resource iterator this case. The PathFilters class includes many static methods to create and compose filters that perform various functions, including finding child paths or exact matches, or matching an Ant-style "glob" pattern.
Additional Code Examples For Filtering Resouces
The following examples demonstrate how to filter resources based on different criteria.

Example 3.14. Find all files named "messages.properties" in your deployment.

ModuleClassLoader moduleClassLoader = (ModuleClassLoader) TargetClass.class.getClassLoader();
Iterator<Resource> mclResources = PathFilters.filtered(PathFilters.match("**/messages.properties"), moduleClassLoader.iterateResources("", true));

Example 3.15. Find all files named "messages.properties" in your deployment and imports.

ModuleClassLoader moduleClassLoader = (ModuleClassLoader) TargetClass.class.getClassLoader();
Module module = moduleClassLoader.getModule();
Iterator<Resource> moduleResources = module.iterateResources(PathFilters.match("**/message.properties));

Example 3.16. Find all files inside any directory named "my-resources" in your deployment.

ModuleClassLoader moduleClassLoader = (ModuleClassLoader) TargetClass.class.getClassLoader();
Iterator<Resource> mclResources = PathFilters.filtered(PathFilters.match("**/my-resources/**"), moduleClassLoader.iterateResources("", true));

Example 3.17. Find all files named "messages" or "errors" in your deployment and imports.

ModuleClassLoader moduleClassLoader = (ModuleClassLoader) TargetClass.class.getClassLoader();
Module module = moduleClassLoader.getModule();
Iterator<Resource> moduleResources = module.iterateResources(PathFilters.any(PathFilters.match("**/messages"), PathFilters.match("**/errors"));

Example 3.18. Find all files in a specific package in your deployment.

ModuleClassLoader moduleClassLoader = (ModuleClassLoader) TargetClass.class.getClassLoader();
Iterator<Resource> mclResources = moduleClassLoader.iterateResources("path/form/of/packagename", false);