Shulou(Shulou.com)06/01 Report--

This article introduces the knowledge of "what is the parent delegation mechanism of java class loading". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

A compiled class file, in order to run in JVM, needs to be loaded into JVM first. In java, the loading tool of a class is abstracted as a class loader, and the specific way of loading class files through the loading tool is called parental delegation mechanism.

Knowledge point

Class loader: a tool that gets its binary (.class) stream by a fully qualified name of a class.

Aforementioned

First of all, it is clear that classes in Java describe the behavior and state of a class of objects and have the characteristics of encapsulation, inheritance and polymorphism. The class loader is a subclass of the abstract class ClassLoader in terms of class structure.

Public abstract class ClassLoader {...}

According to the specific functions, JDK provides a number of specific subclasses, such as ExtClassLoader, AppClassLoader, URLClassLoader, MethodUtil and so on.

To be clear, you have the ability to load classes, but consider actually using them. For example, anyone who knows java knows that the underlying java uses C++. In other words, C++ can also achieve class loading file, can also be said to be a file loading function. This is also done in practical use, so how does the loader of this part of the loaded class be described in Java? Here's the description.

The class loading method actually adopted by JDK is called the parent delegation mechanism. The main categories are as follows, which is an abstract concept.

Bootstrap ClassLoader

The class loader of the system class (rt.ar), loaded by C++ code

Extension ClassLoader

Class loader for extended classes (ext.jar), which is loaded with ExtClassLoader

Application ClassLoader

The classloader of the class on the user classpath (classpath), which is loaded by AppClassLoader

Custom class loader

Custom class loader, which can inherit ClassLoader

Parents

Since it is called the parental appointment mechanism, parents must exist. There is a parent property in the ClassLoader class. So parents can be configured. This parent refers to ExtClassLoader and AppClassLoader, which is set up like this in JDK:

ExtClassLoader.parent=null

AppClassLoader.parent=ExtClassLoader

XXXClassLoader.parent=AppClassLoader

When a custom class loader is built, if parent is not specified, it uses getSystemClassLoader () to get the parent class loader, and this method gets AppClassLoader by default.

The above class loading method is shown below

Delegate

After the parents are set up, you can delegate, and the delegation process is the class file loading process.

Protected Class loadClass (String name, boolean resolve) throws ClassNotFoundException {synchronized (getClassLoadingLock (name)) {/ / guarantee that the class is loaded only once Class c = findLoadedClass (name); if (c = = null) {long t0 = System.nanoTime () Try {if (parent! = null) {/ / if there is a parent classloader, use the parent classloader to load c = parent.loadClass (name, false) } else {/ / there is no parent class loader, then try to use BootstrapClassLoader to load c = findBootstrapClassOrNull (name) }} catch (ClassNotFoundException e) {/ / throw class failed to find exception indicates parent class loader failed to load} if (c = = null) {/ / parent class loader did not finish loading and BootstrapClassLoader did not finish loading, use its own loading method long T1 = System.nanoTime () C = findClass (name); / / this is the defining class loader; record the stats sun.misc.PerfCounter.getParentDelegationTime (). AddTime (T1-T0); sun.misc.PerfCounter.getFindClassTime (). AddElapsedTimeFrom (T1); sun.misc.PerfCounter.getFindClasses (). Increment () }} if (resolve) {resolveClass (c);} return c;}}

Describe this code in the text:

Before loading the class file, verify that it has been loaded, and if it has been loaded, it will no longer be loaded to prevent it from being overwritten and to prevent multiple loads from wasting resources. Then get the parent of the classloader

If parent does not exist, it means that it is already ExtClassLoader. Query whether the next class file has been loaded by Bootstrap ClassLoader, and return if it is found. Otherwise, null is returned.

If parent exists, let the parent class loader load the class file, which is recursive up, such as parent- > parent- > parent

After the above process is processed, if it is found that null is returned, it means that the file has been recursively returned to Bootstrap ClassLoader and still cannot be loaded, which means that this kind of file does not meet the loading conditions of all parent class loaders, so it is your turn to load it.

The process of upward recursion in the above description is delegation, and the way of recursion is parent, so it is called parent delegation mechanism. Of course, the popular understanding is the parent loader priority loading mode.

Postscript

In the delegation process, it is described that the file in this class does not meet the loading conditions of all parent class loaders, so it is your turn to load it. The loading conditions are explained here.

The load folder System.getProperty ("sun.boot.class.path") of Bootstrap ClassLoader is specified in Launcher; this is mainly the\ lib\ rt package specified by JDK

The load folder is specified in ExtClassLoader: String var0 = System.getProperty ("java.ext.dirs"); this is mainly the\ lib\ ext package specified by JDK

The load folder is specified in AppClassLoader: String var1 = System.getProperty ("java.class.path"); all are environment variables and can be configured on their own. This mainly refers to the classpath in development.

Since both AppClassLoader and ExtClassLoader inherit URLClassLoader, they both determine the loading condition. If it is custom, you can rewrite loadClass to bypass the parent delegation. Of course, no matter how overridden, the method of loading a binary stream verifies that the class file cannot load classes in a package that starts with java, which cannot be bypassed.

Custom class loader example

Code

Public class ConsumerClassLoaderDemo extends ClassLoader {public static void main (String [] args) throws Exception {ClassLoader myClassLoader = new ConsumerClassLoader (); Object obj = myClassLoader.loadClass ("cn.tinyice.demo.classloader.ConsumerClassLoaderDemo"). NewInstance (); ClassLoader classLoader = obj.getClass (). GetClassLoader (); / / BootStrapClassLoader does not exist in Java, so it will be null while (null! = classLoader) {System.out.println (classLoader) ClassLoader = classLoader.getParent ();}} class ConsumerClassLoader extends ClassLoader {@ Override public Class loadClass (String name) throws ClassNotFoundException {try {String classFile = name.substring (name.lastIndexOf (".") + 1) + ".class"; InputStream in = getClass () .getResourceAsStream (classFile); if (null = = in) {return super.loadClass (name) } byte [] bytes = new byte [in.available ()]; in.read (bytes); return defineClass (name, bytes, 0, bytes.length);} catch (IOException e) {throw new ClassNotFoundException (name);}

Console

That's all for cn.tinyice.demo.classloader.ConsumerClassLoader@12133b1sun.misc.Launcher$AppClassLoader@b4aac2sun.misc.Launcher$ExtClassLoader@f49f1cProcess finished with exit code 0, "what is the parent delegation mechanism loaded by the java class?" Thank you for reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.