The class loader (Class Loader) in Java is one of the important components of the Java Virtual Machine (JVM). When the Java program is executed, the class loader is responsible for loading the compiled Java code (that is, the .class file) from the disk into the memory, and creating the corresponding Class object, so that the program can use these classes to create objects, call methods, etc. .
The main function of the class loader is to dynamically load classes at runtime, which makes Java programs highly flexible and scalable. Class loaders are generally regarded as an important feature of the Java language, which enables Java programs to be developed and deployed in a plug-in manner without recompiling and deploying the entire application.
Class loaders in Java are usually divided into three main components: bootstrap class loader (Bootstrap Class Loader), extension class loader (Extension Class Loader) and system class loader (System Class Loader). Each class loader has its own loading path and loading order, and they can cooperate with each other to load classes.
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Bootstrap class loader
The bootstrap class loader is a part inside the JVM, which is mainly responsible for loading Java's core class library, such as the classes in the java.lang package. It is the top-level class loader in the Java class loader, and it is usually implemented in native code, and its instance cannot be obtained through Java code. -
Extended class loader
The extended class loader is responsible for loading the classes in the JVM extension directory, usually the classes in the jre/lib/ext directory. The extended class loader is implemented by Java code, which inherits from the java.lang.ClassLoader class. -
System class loader
The system class loader, also known as the application class loader, is responsible for loading application classes, usually classes under the class path specified in the CLASSPATH environment variable. The system class loader is also implemented by Java code, which inherits from the java.lang.ClassLoader class.
The loading order of the class loader is usually: the bootstrap class loader loads the core class library, the extension class loader loads the extension library, and the system class loader loads the application class. When a class needs to be loaded, the JVM will first check whether the class already exists in the core class library through the bootstrap class loader, and if found, it will directly return the Class object. If not found, the extended class loader will check whether the class exists in the extended library, and return the Class object if found. Finally, if the extended class loader does not find the class, the system class loader checks whether the class exists in the application class path, and returns the Class object if found.
The role of class loaders in Java is very important. They can not only dynamically load classes, but also realize plug-in development and deployment of Java programs. The class loader can implement a custom class loading method by overriding the findClass() method of ClassLoader, so as to meet various needs. For example, you can protect code security by customizing class loaders, implement hot deployment of classes, or implement special class loading methods, etc.
In practical applications, class loaders are often used to resolve class version conflicts. For example, in a web application, multiple class libraries of different versions may be referenced, and these class libraries may have the same class name and package name. To solve this problem, different class loaders can be used to load different versions of class libraries, thereby avoiding class conflicts.
In addition, the class loader can also realize the dynamic expansion of the Java program, allowing the dynamic loading and unloading of the class library at runtime, thereby realizing the ability to dynamically add and delete functional modules.
In general, the class loader is an important feature of the Java language, which provides high flexibility and scalability for Java programs. Understanding the working principle and usage of class loaders is very important for Java developers, which can help them better understand the operating mechanism of the Java language and better develop and deploy Java programs.