How Java implements the parent delegate loading mechanism

Java is an object-oriented programming language, it has a unique class loading mechanism. Among them, the parent delegation loading mechanism is an important concept in the Java class loading mechanism. This article describes how Java's parent delegate loading mechanism is implemented, and explains its role and advantages.

Java class loading mechanism

In Java, class loading occurs at runtime. When a program needs to use a certain class, the JVM will first check whether the class has been loaded. If not loaded, the bytecode file of the class will be loaded into the memory, and the corresponding class object will be generated in the memory for use by the program. Class loading can be achieved through the ClassLoader class.

Java's class loading mechanism is implemented based on the delegation mechanism. When a class needs to be loaded, the JVM will first entrust the task to the parent loader for loading, and will not hand over the task to the child loader for loading until the parent loader cannot load the class. If none of the parent loaders can load the class, the current loader will eventually load the class.

Parent delegate loading mechanism

The parent delegation loading mechanism means that when a class loader loads a class, it first entrusts the task to the parent class loader for loading. Only when the parent class loader cannot load the class, the current class loader will load it. This mechanism ensures the uniqueness and security of the class, and also avoids repeated loading of the class.

In Java, the implementation of the parent delegate loading mechanism is done through the ClassLoader class. The ClassLoader class is an abstract class that defines three methods: loadClass(), findClass(), and defineClass(). Among them, the loadClass() method is the core method to implement the parent delegation loading mechanism.

Implementation of the loadClass() method

The loadClass() method is the core method of the ClassLoader class, which implements the parent delegate loading mechanism. In the loadClass() method, it is first checked whether the class has been loaded. If it is already loaded, return the class directly. If not loaded, the loading task of the class is delegated to the parent class loader for loading. If the parent class loader cannot load the class, it loads it itself.

Here is a simple implementation of the loadClass() method:

public class MyClassLoader extends ClassLoader {
    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        // 检查该类是否已经被加载
        Class<?> clazz = findLoadedClass(name);
        if (clazz != null) {
            return clazz;
        }

        // 委托给父类加载器进行加载
        ClassLoader parent = getParent();
        try {
            clazz = parent.loadClass(name);
            return clazz;
        } catch (ClassNotFoundException e) {
            // 父类加载器无法加载该类，则自己进行加载
            return findClass(name);
        }
    }
}

In the above code, the loadClass() method first calls the findLoadedClass() method to check whether the class has been loaded. If it is already loaded, return the class directly. Otherwise, the task of loading the class is delegated to the parent class loader for loading. If the parent class loader cannot load the class, it loads it itself.

advantage

Java's parent delegate loading mechanism has the following advantages:

Avoid duplicate loading of classes

In the parent delegation loading mechanism, each class loader has a unique parent class loader. When a class needs to be loaded, it is first delegated to the parent class loader for loading. If the parent class loader has already loaded the class, the class loaded by the parent class loader is returned directly. This avoids repeated loading of classes and saves memory space.

Ensure class uniqueness

In the parent delegation loading mechanism, the loading order of classes is from top to bottom. In other words, the child class loader will delegate to the parent class loader for loading. In this way, the uniqueness of the class is ensured, the confusion of the class is avoided, and the security of the program is guaranteed.

Clear order of loading

In the parent delegation loading mechanism, the loading order of classes is from top to bottom, that is, the parent class loader loads first, and then the child class loader loads. This ensures that the loading order of classes is clear and avoids class confusion.

Summarize

Java's parent delegation loading mechanism is an important class loading mechanism. It uses the delegation mechanism to ensure the uniqueness and security of classes, avoid repeated loading of classes, and ensure the performance and security of programs. In Java, the implementation of the parental delegation loading mechanism is accomplished through the ClassLoader class, and the loadClass() method is the core method for realizing the parental delegation loading mechanism. When using a class loader, the parent delegate loading mechanism should be followed to ensure the correctness and security of the program.