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This article focuses on "how to destroy the Java parent delegation model", interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "how to destroy the Java parent delegation model".

Custom example 1

Rewriting findClass does not destroy the parent delegation model, but also verifies the correctness of the parent delegation model.

Package com.example.classloader;import org.junit.Test;import java.io.File;import java.io.FileInputStream;import java.io.FileNotFoundException;import java.io.IOException;public class TestClassLoader {@ Test public void test () {Class class0 = TestClassLoader.class; try {/ / false System.out.println (class0.getClassLoader () instanceof MyClassLoader) Class class1 = class0.getClassLoader () .loadClass ("com.example.classloader.TestClassLoader"); ClassLoader classLoader = new MyClassLoader (); Class class2 = classLoader.loadClass ("com.example.classloader.TestClassLoader"); / / true System.out.println (class0.equals (class1)) / true System.out.println (class1.equals (class2)); / / sun.misc.Launcher$AppClassLoader@18b4aac2 System.out.println (class0.getClassLoader ()); / / sun.misc.Launcher$AppClassLoader@18b4aac2 System.out.println (class1.getClassLoader ()) / / sun.misc.Launcher$AppClassLoader@18b4aac2 System.out.println (class2.getClassLoader ());} catch (ClassNotFoundException e) {e.printStackTrace () }} / * customize a class loader to load classes from the specified disk directory * overriding findClass does not break the parent delegation model * / public class MyClassLoader extends ClassLoader {@ Override protected Class findClass (String name) { String myPath = "/ Users/lsx/code/demo/target/classes/" + name.replace ("." "/") + ".class" Byte [] classBytes = null; FileInputStream in = null; try {File file = new File (myPath); in = new FileInputStream (file); classBytes = new byte [(int) file.length ()] In.read (classBytes);} catch (FileNotFoundException e) {e.printStackTrace ();} catch (IOException e) {e.printStackTrace () } finally {try {in.close ();} catch (IOException e) {e.printStackTrace () }} Class clazz = defineClass (name, classBytes, 0, classBytes.length); return clazz;}} custom example 2

Rewriting loadClass destroys the parent delegate model, and the Object of the parent class is also loaded by our custom class loader. Obviously, there will be no Object.class file in our custom load directory, so an exception will be thrown.

/ * rewrite loadClass to destroy the parental delegation model * java.io.FileNotFoundException: / Users/lsx/code/demo/target/classes/java/lang/Object.class (No such file or directory) * / public class MyClassLoader extends ClassLoader {@ Override public Class loadClass (String name) {String myPath = " / Users/lsx/code/demo/target/classes/ "+ name.replace (". " "/") + ".class" System.out.println (myPath); byte [] classBytes = null; FileInputStream in = null; try {File file = new File (myPath); in = new FileInputStream (file) ClassBytes = new byte [(int) file.length ()]; in.read (classBytes);} catch (FileNotFoundException e) {e.printStackTrace () } catch (IOException e) {e.printStackTrace ();} finally {try {in.close () } catch (IOException e) {e.printStackTrace ();}} System.out.println (); Class clazz = defineClass (name, classBytes, 0, classBytes.length) Return clazz;}}

Summary: if you don't want to break the parent delegation model, override the findClass () method in the ClassLoader class, and classes that cannot be loaded by the parent class loader will eventually be loaded through this method. If you want to break the parental delegation model, you need to override the loadClass () method.

JDBC's destruction of the parent delegation model (loading the SPI interface implementation class)

In the native JDBC, the Driver driver itself is only an interface, and there is no specific implementation, the specific implementation is implemented by different database types. For example, the Driver class in MySQL's mysql-connector-.jar is specifically implemented. The classes in the native JDBC are placed in the rt.jar package and loaded by the startup class loader. In the Driver class in JDBC, Driver classes of different database types need to be loaded dynamically, while the Driver class in mysql-connector-.jar is user-written code, so the startup class loader must not be loaded. Since it is your own code, you need the application startup class to load the class. Therefore, the thread upper and lower file class loader (Thread Context ClassLoader) is introduced at this time. With this, the program can load the classes that need to be loaded by the startup class loader by the application class loader. Through the thread context loader to load the required SPI code, that is, the parent class loader requests the subclass loader to complete the action of class loading, it actually uses the class loader in reverse by breaking the hierarchical structure of the parent delegation model, which violates the general principle of the parent delegation model. Taking JDBC as an example to discuss the destruction of parent delegation model

The destruction of the parental delegation model by OSGI

The third "destruction" of the parental delegation model is caused by the user's pursuit of the dynamic nature of the program. In order to achieve hot plug, hot deployment, modularization, it means to add a function or subtract a function without restarting, just replace the module with similar loaders to achieve the hot replacement of the code. For example, with the emergence of OSGi, in the OSGi environment, the classloader is no longer a tree structure in the parent delegation model, but further develops into a network structure.

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