How to use the ClassLoader function for dynamic class loading in Java

The ClassLoader function in Java can dynamically load classes at runtime, which is very useful in some applications that require flexible deployment and modification of code. Through the function of ClassLoader, the plug-in mechanism can be implemented to improve the scalability and flexibility of the system. This article will introduce how to use the ClassLoader function for dynamic class loading.

1. The role of ClassLoader

When starting the Java virtual machine (JVM), three ClassLoaders will be created: startup class loader, extension class loader and system class loader. The startup class loader is used to load Java core libraries, the extension class loader is used to load extension libraries, and the system class loader is used to load application classes and dependent class libraries.

The function of ClassLoader is to load the specified class file through the specified path and convert it into a Class object. ClassLoader is divided into two categories: system class loader and custom class loader. The system class loader can load class files in the ClassPath environment and class files in dependent jar packages; while the custom class loader can load class files from the network or database according to requirements and convert them into Class objects.

2. How to use ClassLoader to load classes

1. Class relationship of ClassLoader

Before loading a class, we need to understand the class relationship of ClassLoader. ClassLoader is an abstract class that inherits the Object class and has several subclasses: URLClassLoader, AppClassLoader, ExtClassLoader, etc. Among them, URLClassLoader is the most commonly used class loader, which allows us to load class files from a specified path (including network paths).

2. ClassLoader’s API

ClassLoader’s API includes two important methods:

(1) loadClass(String name): This method is ClassLoader The core method is used to load the class with the specified class name. If the ClassLoader has already loaded the class, it will directly return the Class object of the class. Otherwise, this method will entrust the parent ClassLoader to load it step by step. If the class is not found, the ClassLoader will load it by itself and replace the binary code of the class. Convert to Class object.

(2) findClass(String name): This method is a method that subclasses of ClassLoader need to implement to load classes from the specified path. This method must find the binary code according to the specified class name and call the defineClass method to convert it into a Class object.

3. Custom ClassLoader

If you need to implement a custom ClassLoader, we need to follow the following steps:

(1) Inherit the ClassLoader class.

(2) Implement the findClass method, which is used to find the class file from the specified path and return the binary code of the file.

(3) Call the defineClass method to convert the binary code into a Class object.

The following is a simple custom ClassLoader code example:

class MyClassLoader extends ClassLoader {

    private String classpath;

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

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] data = getClassData(name);
        return defineClass(name, data, 0, data.length);
    }

    private byte[] getClassData(String name) throws ClassNotFoundException {
        //TODO 从classpath中查找类文件，并返回二进制代码
        return null;
    }

}

4. Dynamic loading class

With a custom ClassLoader, we can Dynamically load classes during program running. The following is a simple code example:

public class Test {

    public static void main(String[] args) throws Exception {
        String classpath = "文件路径";
        MyClassLoader myClassLoader = new MyClassLoader(classpath);
        Class<?> clazz = myClassLoader.loadClass("类名");
        Object obj = clazz.newInstance();
        Method method = clazz.getMethod("方法名");
        method.invoke(obj);
    }

}

In the above code, we dynamically load the specified class through MyClassLoader, and then call the method of the class through the reflection mechanism.

Summary

Through Java's ClassLoader mechanism, we can realize the need to dynamically load classes at runtime and improve the flexibility and scalability of the program. A custom ClassLoader needs to inherit the ClassLoader class and implement the findClass method, and then use the ClassLoader to dynamically load classes in the program.

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