Java ClassLoader Loading mechanism understanding practical examples

For the Java classloader loading mechanism, we made a simple classloader and successfully loaded the specified class.

No nonsense, just on the code.

 Packagecom.chq.study.cl;ImportJava.io.ByteArrayOutputStream;ImportJava.io.File;ImportJava.io.FileInputStream;Importjava.io.FileNotFoundException;Importjava.io.IOException;ImportJava.nio.ByteBuffer;ImportJava.nio.channels.Channels;ImportJava.nio.channels.FileChannel;ImportJava.nio.channels.WritableByteChannel;/*** @desc Custom ClassLoader, can only load the. class end, to test the Java ClassLoader mechanism*/ Public classChqclassloaderextendsClassLoader {PrivateString FileName;  PublicChqclassloader (String fileName) { This. FileName =FileName; }    protectedClass<?> Findclass (String className)throwsclassnotfoundexception {Class<?> Clazz = This. Findloadedclass (ClassName); if(NULL==clazz) {            Try{String Classfile=Getclassfile (className); System.out.println ("Findclass" +classfile); FileInputStream FIS=NewFileInputStream (Classfile); FileChannel Filec=Fis.getchannel (); Bytearrayoutputstream BAOs=NewBytearrayoutputstream (); Writablebytechannel OUTC=Channels.newchannel (BAOs); Bytebuffer Buffer= Bytebuffer.allocatedirect (1024);  while(true) {                    inti =filec.read (buffer); if(i = = 0 | | i = =-1) {                         Break;                    } buffer.flip ();                    Outc.write (buffer);                Buffer.clear ();                } fis.close (); byte[] bytes =Baos.tobytearray (); Clazz= DefineClass (className, Bytes, 0, bytes.length); } Catch(FileNotFoundException e) {e.printstacktrace (); } Catch(IOException e) {e.printstacktrace (); }        }        returnClazz; }    Privatestring Getclassfile (string name) {StringBuffer SB=NewStringBuffer (fileName); Name= Name.replace ('. ', File.separatorchar) + ". Class"; Sb.append (File.separator+name); returnsb.tostring (); }}

 package   com.chq.study.cl;  /**   * @desc Self Introduction Test interface class, plastic type with the  */ public  interface   ITest { void   self ();}  

 Package com.chq.study.cl; /**  */Publicclass  Test    {publicvoid self () {        System.out.println (this);}    }

 Packagecom.chq.study.cl;/*** @desc Self Introduction Test implementation class*/ Public classTestimplImplementsITest {/*(non-javadoc) * @see com.chq.study.cl.itest#self ()*/@Override Public voidSelf () {System.out.println ("This was from Testimpl instance" + This); }} 

 Packagecom.chq.study.cl;/** * @authorchenqing * @datetime February 4, 2015 PM 4:54:12 * @desc Entry class, call the custom ClassLoader to load the class for validation*/ Public classMainclassloader {/**     * @paramargs*/     Public Static voidMain (string[] args) {Chqclassloader cl=NewChqclassloader ("C:\\workspaces\\myeclipse Professional 2014\\classloader\\bin"); Try{Class<?> clazz = Cl.findclass ("Com.chq.study.cl.Test"); Try {                //execution here throws an exception, validating the ClassLoader's overall accountability mechanismTest cc =(Test) clazz.newinstance ();                Cc.self (); System.out.println ("Belong class Loader:" +Cc.getclass (). getClassLoader (). toString ()); } Catch(instantiationexception e) {e.printstacktrace (); } Catch(illegalaccessexception e) {e.printstacktrace (); } Catch(classcastexception e) {e.printstacktrace (); }                        //ensure output order            Try{Thread.Sleep (100); } Catch(interruptedexception E1) {e1.printstacktrace (); } clazz= Cl.findclass ("Com.chq.study.cl.TestImpl"); Try {                //normal here, bypassing the overall responsibility mechanism through plasticity as a base class.ITest IC =(ITest) clazz.newinstance ();                Ic.self (); System.out.println ("Belong class Loader:" +Ic.getclass (). getClassLoader (). toString ()); } Catch(instantiationexception e) {e.printstacktrace (); } Catch(illegalaccessexception e) {e.printstacktrace (); } Catch(classcastexception e) {e.printstacktrace (); }        } Catch(ClassNotFoundException e) {e.printstacktrace (); }    }}

The results of the final output show that Test & Testimpl are found, but when instantiated, the former fails because of different classloader loads, while the latter is successfully loaded and instantiated by the molded type as the base class.

Findclass C:\workspaces\MyEclipse Professional 2014\classloader\bin\com\chq\study\cl\test.class  Java.lang.ClassCastException:com.chq.study.cl.Test cannot is cast to com.chq.study.cl.Test at    Com.chq.study.cl.MainClassLoader.main (Mainclassloader.java:2014\classloader\bin\com\chq\study\ Cl\testimpl. class  This  Class loader: [Email protected]

Java ClassLoader Loading mechanism understanding practical examples