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3.6. Use the Class Loader Programmatically in a Deployment

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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
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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());

Class<?> targetClass = Class.forName("com.myorg.util.TargetClass", true, CurrentClass.class.getClassLoader());

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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 JDK 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");

InputStream inputStream = CurrentClass.class.getResourceAsStream("targetResourceName");

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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
            }
        }
    }
}

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
            }
        }
    }
}

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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");

InputStream inputStream = CurrentClass.class.getResourceAsStream(TargetClass.class.getSimpleName() + ".class");

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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");

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

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3.6.1. Programmatically Load Classes and Resources in a Deployment
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