An example of an in-depth understanding of ClassLoader

The file class loader, which loads the LoadClass method, logically reads only the file to load the class, not the parent ClassLoader to load

 PackageCom.ydd.study.hello.classloader;ImportJava.io.ByteArrayOutputStream;ImportJava.io.File;ImportJava.io.FileInputStream;Importjava.io.IOException;ImportJava.io.InputStream; Public classFilesystemclassloaderextendsClassLoader {PrivateString RootDir;  PublicFilesystemclassloader (String rootdir) { This. RootDir =RootDir; }     protectedClass<?> Findclass (String name)throwsClassNotFoundException {byte[] Classdata =getclassdata (name); if(Classdata = =NULL) {             Throw Newclassnotfoundexception (); }         Else {             returnDefineClass (name, Classdata, 0, classdata.length); }} @Override PublicClass<?>loadclass (String name) {Try {            returnFindclass (name); } Catch(ClassNotFoundException e) {//TODO auto-generated Catch blockE.printstacktrace (); return NULL; }                    }    Private byte[] Getclassdata (String className) {string path=Classnametopath (className); Try{InputStream ins=NewFileInputStream (path); Bytearrayoutputstream BAOs=NewBytearrayoutputstream (); intBufferSize = 4096; byte[] buffer =New byte[buffersize]; intBytesnumread = 0;  while((Bytesnumread = ins.read (buffer))! =-1) {baos.write (buffer,0, Bytesnumread); }             returnBaos.tobytearray (); } Catch(IOException e) {e.printstacktrace (); }         return NULL; }     Privatestring Classnametopath (String className) {returnRootDir +File.separatorchar+ classname.replace ('. ', File.separatorchar) + ". Class"; } }

Test code

 PackageCom.ydd.study.hello.classloader;Importorg.junit.Test; Public classfilesystemclassloadertest {@Test Public voidTest () {Filesystemclassloader Filesystemclassloader=NewFilesystemclassloader ("d:\\"); Try{Class<?>c=filesystemclassloader.loadclass ("TestClass");            System.out.println (c); Object TestClass=(C.newinstance ()); System.out.println (c+"|" +TestClass); } Catch(Exception e) {//TODO auto-generated Catch blockE.printstacktrace (); }            }}

From the output below, you can see:

1. When loading the TestClass class, it has already caused other classes to load.

2, do not easily overload loadclass (), parental system is very important, otherwise it will cause a class to exist multiple class objects.

3, Class A depends on the loading of other classes, all of the classloader of Class A to initiate the initial loading

Java.io.filenotfoundexception:d:\java\lang\object.class (the system cannot find the path specified.) ) at Java.io.FileInputStream.open (Native Method) at java.io.fileinputstream.<init> (fileinputstream.java:146) At Java.io.fileinputstream.<init> (fileinputstream.java:101) at Com.ydd.study.hello.classloader.FileSystemClassLoader.getClassData (filesystemclassloader.java:42) at Com.ydd.study.hello.classloader.FileSystemClassLoader.findClass (filesystemclassloader.java:17) at Com.ydd.study.hello.classloader.FileSystemClassLoader.loadClass (filesystemclassloader.java:29) at Java.lang.ClassLoader.defineClass1 (Native Method) at Java.lang.ClassLoader.defineClass (classloader.java:800) at Java.lang.ClassLoader.defineClass (classloader.java:643) at Com.ydd.study.hello.classloader.FileSystemClassLoader.findClass (filesystemclassloader.java:22) at Com.ydd.study.hello.classloader.FileSystemClassLoader.loadClass (filesystemclassloader.java:29) at Com.ydd.study.hello.classloader.FileSystemClassLoaderTest.test (filesystemclassloadertest.java:12) at Sun.reflect.NativeMethodAccessorImpl.invoke0 (Native Method) at Sun.reflect.NativeMethodAccessorImpl.invoke ( nativemethodaccessorimpl.java:57) at Sun.reflect.DelegatingMethodAccessorImpl.invoke ( delegatingmethodaccessorimpl.java:43) at Java.lang.reflect.Method.invoke (method.java:606) at Org.junit.runners.model.frameworkmethod$1.runreflectivecall (frameworkmethod.java:47) at Org.junit.internal.runners.model.ReflectiveCallable.run (Reflectivecallable.java:12) at Org.junit.runners.model.FrameworkMethod.invokeExplosively (frameworkmethod.java:44) at Org.junit.internal.runners.statements.InvokeMethod.evaluate (invokemethod.java:17) at Org.junit.runners.ParentRunner.runLeaf (parentrunner.java:271) at Org.junit.runners.BlockJUnit4ClassRunner.runChild (blockjunit4classrunner.java:70) at Org.junit.runners.BlockJUnit4ClassRunner.runChild (BLOCKJUNIT4CLASSRUNNER.JAVA:50) at Org.junit.runners.parentrunner$3.run (parentrunner.java:238) at Org.junit.runners.parentrunner$1.schedule ( parentrunner.java:63) at org.junit.ruNners. Parentrunner.runchildren (parentrunner.java:236) at org.junit.runners.parentrunner.access$000 (ParentRunner.java : @ org.junit.runners.parentrunner$2.evaluate (parentrunner.java:229) at Org.junit.runners.ParentRunner.run ( parentrunner.java:309) at Org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run ( JUNIT4TESTREFERENCE.JAVA:50) at Org.eclipse.jdt.internal.junit.runner.TestExecution.run (testexecution.java:38) at Org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests (remotetestrunner.java:459) at Org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests (remotetestrunner.java:675) at Org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run (remotetestrunner.java:382) at Org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main (remotetestrunner.java:192) Java.lang.ClassNotFoundExceptionat Com.ydd.study.hello.classloader.FileSystemClassLoader.findClass ( filesystemclassloader.java:19) at Com.ydd.study.hello.classloader.FileSystemClassLoader.loadClass ( Filesystemclassloader.java:29) at Java.lang.ClassLoader.defineClass1 (Native Method) at Java.lang.ClassLoader.defineClass ( classloader.java:800) at Java.lang.ClassLoader.defineClass (classloader.java:643) at Com.ydd.study.hello.classloader.FileSystemClassLoader.findClass (filesystemclassloader.java:22) at Com.ydd.study.hello.classloader.FileSystemClassLoader.loadClass (filesystemclassloader.java:29) at Com.ydd.study.hello.classloader.FileSystemClassLoaderTest.test (Filesystemclassloadertest.java:12) at SUN.REFLECT.NATIVEMETHODACCESSORIMPL.INVOKE0 (Native Method) at Sun.reflect.NativeMethodAccessorImpl.invoke ( nativemethodaccessorimpl.java:57) at Sun.reflect.DelegatingMethodAccessorImpl.invoke ( delegatingmethodaccessorimpl.java:43) at Java.lang.reflect.Method.invoke (method.java:606) at Org.junit.runners.model.frameworkmethod$1.runreflectivecall (frameworkmethod.java:47) at Org.junit.internal.runners.model.ReflectiveCallable.run (Reflectivecallable.java:12) at Org.junit.runners.model.FrameworkMethod.invokeExplosively (Frameworkmethod.java:44) at Org.junit.internal.runners.statements.InvokeMethod.evaluate (invokemethod.java:17) at Org.junit.runners.ParentRunner.runLeaf (parentrunner.java:271) at Org.junit.runners.BlockJUnit4ClassRunner.runChild (blockjunit4classrunner.java:70) at Org.junit.runners.BlockJUnit4ClassRunner.runChild (BLOCKJUNIT4CLASSRUNNER.JAVA:50) at Org.junit.runners.parentrunner$3.run (parentrunner.java:238) at Org.junit.runners.parentrunner$1.schedule ( parentrunner.java:63) at Org.junit.runners.ParentRunner.runChildren (parentrunner.java:236) at org.junit.runners.parentrunner.access$000 (parentrunner.java:53) at Org.junit.runners.parentrunner$2.evaluate ( parentrunner.java:229) at Org.junit.runners.ParentRunner.run (parentrunner.java:309) at Org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run (JUNIT4TESTREFERENCE.JAVA:50) at Org.eclipse.jdt.internal.junit.runner.TestExecution.run (testexecution.java:38) at Org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests (remotetestrunner.java:459) at ORG.ECLIPSE.JDt.internal.junit.runner.RemoteTestRunner.runTests (remotetestrunner.java:675) at Org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run (remotetestrunner.java:382) at Org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main (remotetestrunner.java:192)

　　

An example of an in-depth understanding of ClassLoader