How to use the ClassLoader function for class loading in Java

The principle and method of using the ClassLoader function for class loading in Java has always been one of the focuses of Java developers. The ClassLoader function is part of the Java class library. Its main function is to load Java class files into the Java Virtual Machine (JVM) so that the program can run normally. The ClassLoader function is the core of Java class loading. In the Java runtime environment, it is responsible for finding and loading the bytecode of Java classes. Therefore, it is crucial for Java programmers to understand and master the usage and principles of the ClassLoader function. of.

The ClassLoader function in Java is divided into three levels, namely Bootstrap ClassLoader, Extention ClassLoader and Application ClassLoader. Bootstrap ClassLoader is the internal implementation of the JVM, which is responsible for loading classes in the Java core API library into the JVM. The Extention ClassLoader layer is implemented by the extension API provided by the Java SDK. Application ClassLoader is a ClassLoader implemented by the application itself and is used to load application code.

In Java programs, ClassLoader is composed of a Hierarchy structure, and its parent-child relationship is established by the java.lang.ClassLoader class. The main methods defined by this class are findClass(String) and loadClass(String), through which the class can be loaded.

In Java, ClassLoader is a class that implements class loading by inheriting two methods: findClass(String name) and loadClass(String name). When a program calls a class, ClassLoader first searches the JVM to see if the class has been loaded. If it has been loaded, it returns the class directly. Otherwise, ClassLoader starts looking for the class file. The search method of ClassLoader is implemented by calling the findClass method. findClass first calls the search method of the java.lang.ClassLoader class. By default, the findClass method returns NullPointerException because the default ClassLoader cannot find class files. If we want to implement loading classes ourselves method, you must override the findClass method of ClassLoader and implement your own loading logic.

Below, we introduce the use of the ClassLoader function in detail through a class loading example.

Examples are as follows:

We customize the ClassLoader class to achieve functions that cannot be achieved by the system ClassLoader.

The code of the custom ClassLoader class is as follows:

public class MyClassLoader extends ClassLoader {

    private String classPath;

    public MyClassLoader(String classPath) {
        this.classPath = classPath;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classData = getData(name);
        if (classData == null) {
            throw new ClassNotFoundException();
        } else {
            return defineClass(name, classData, 0, classData.length);
        }
    }

    private byte[] getData(String className) {
        String path = classPath + File.separatorChar + className.replace('.', File.separatorChar) + ".class";
        try (InputStream is = new FileInputStream(path);
             ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
            int bufferSize = 1024;
            byte[] buffer = new byte[bufferSize];
            int length = -1;
            while ((length = is.read(buffer)) != -1) {
                baos.write(buffer, 0, length);
            }
            return baos.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }
}

In the code, we inherit the ClassLoader class, overwrite the findClass method, and realize the search and loading of the class ourselves. The getData method is a private method used to read binary data in class files. Here, we use the FileInputStream and ByteArrayOutputStream classes of Java IO to convert the read file-like data into a byte array.

The code for using the custom ClassLoader class is as follows:

public class Test {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
        MyClassLoader loader = new MyClassLoader("D:\class");
        Class c = loader.loadClass("com.test.Test");
        Object obj = c.newInstance();
        System.out.println(obj);
    }
}

In this example, we load the com.test.Test class from the specified path through the custom ClassLoader class, and then create the class instance and print it out.

To summarize, using the ClassLoader function to load classes in Java is mainly divided into three steps: customizing the ClassLoader class, overriding the findClass method, and using the custom ClassLoader class to implement class loading. Since the class loading mechanism in Java is a very important mechanism, it is crucial for Java programmers to master and deeply understand the usage and principles of the ClassLoader function.

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