【转】Java类加载原理解析

4常见问题分析：

4.1由不同的类加载器加载的指定类型还是相同的类型吗？

4.2在代码中直接调用Class.forName（Stringname）方法，到底会触发那个类加载器进行类加载行为？

//java.lang.Class.java
publicstaticClass<?>forName(StringclassName)throwsClassNotFoundException{
returnforName0(className,true,ClassLoader.getCallerClassLoader());
}

//java.lang.ClassLoader.java
//Returnstheinvoker'sclassloader,ornullifnone.
staticClassLoadergetCallerClassLoader(){
//获取调用类（caller）的类型
Classcaller=Reflection.getCallerClass(3);
//ThiscanbenulliftheVMisrequestingit
if(caller==null){
returnnull;
}
//调用java.lang.Class中本地方法获取加载该调用类（caller）的ClassLoader
returncaller.getClassLoader0();
}

//java.lang.Class.java
//虚拟机本地实现，获取当前类的类加载器，前面介绍的Class的getClassLoader()也使用此方法
nativeClassLoadergetClassLoader0();

4.3在编写自定义类加载器时，如果没有设定父加载器，那么父加载器是？

protectedClassLoader(){

SecurityManagersecurity=System.getSecurityManager();

if(security!=null){

security.checkCreateClassLoader();

}

this.parent=getSystemClassLoader();

initialized=true;

}

privatestaticsynchronizedvoidinitSystemClassLoader(){

//...

sun.misc.Launcherl=sun.misc.Launcher.getLauncher();

scl=l.getClassLoader();

//...
}

4.4在编写自定义类加载器时，如果将父类加载器强制设置为null，那么会有什么影响？如果自定义的类加载器不能加载指定类，就肯定会加载失败吗？

4.5编写自定义类加载器时，一般有哪些注意点？

//用户自定义类加载器WrongClassLoader.Java（覆写loadClass逻辑）

publicclassWrongClassLoaderextendsClassLoader{

publicClass<?>loadClass(Stringname)throwsClassNotFoundException{

returnthis.findClass(name);

}

protectedClass<?>findClass(Stringname)throwsClassNotFoundException{

//假设此处只是到工程以外的特定目录Ｄ：/library下去加载类

具体实现代码省略

}
}

publicclassWrongClassLoaderTest{

publicstaticvoidmain(String[]args){

try{

WrongClassLoaderloader=newWrongClassLoader();

ClassclassLoaded=loader.loadClass("beans.Account");

System.out.println(classLoaded.getName());

System.out.println(classLoaded.getClassLoader());

}catch(Exceptione){

e.printStackTrace();

}

}
}

java.io.FileNotFoundException:D:"classes"java"lang"Object.class(系统找不到指定的路径。)

atjava.io.FileInputStream.open(NativeMethod)

atjava.io.FileInputStream.<init>(FileInputStream.java:106)

atWrongClassLoader.findClass(WrongClassLoader.java:40)

atWrongClassLoader.loadClass(WrongClassLoader.java:29)

atjava.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)

atjava.lang.ClassLoader.defineClass1(NativeMethod)

atjava.lang.ClassLoader.defineClass(ClassLoader.java:620)

atjava.lang.ClassLoader.defineClass(ClassLoader.java:400)

atWrongClassLoader.findClass(WrongClassLoader.java:43)

atWrongClassLoader.loadClass(WrongClassLoader.java:29)

atWrongClassLoaderTest.main(WrongClassLoaderTest.java:27)

Exceptioninthread"main"java.lang.NoClassDefFoundError:java/lang/Object

atjava.lang.ClassLoader.defineClass1(NativeMethod)

atjava.lang.ClassLoader.defineClass(ClassLoader.java:620)

atjava.lang.ClassLoader.defineClass(ClassLoader.java:400)

atWrongClassLoader.findClass(WrongClassLoader.java:43)

atWrongClassLoader.loadClass(WrongClassLoader.java:29)

atWrongClassLoaderTest.main(WrongClassLoaderTest.java:27)

//用户自定义类加载器WrongClassLoader.Java(不覆写loadClass逻辑)

publicclassWrongClassLoaderextendsClassLoader{

protectedClass<?>findClass(Stringname)throwsClassNotFoundException{

//假设此处只是到工程以外的特定目录Ｄ：/library下去加载类

具体实现代码省略

}
}

4.6如何在运行时判断系统类加载器能加载哪些路径下的类？

4.7如何在运行时判断标准扩展类加载器能加载哪些路径下的类？

try{
URL[]extURLs=((URLClassLoader)ClassLoader.getSystemClassLoader().getParent()).getURLs();

for(inti=0;i<extURLs.length;i++){

System.out.println(extURLs[i]);

}

}catch(Exceptione){//…}

5开发自己的类加载器

5.1文件系统类加载器

packageclassloader;

importjava.io.ByteArrayOutputStream;
importjava.io.File;
importjava.io.FileInputStream;
importjava.io.IOException;
importjava.io.InputStream;

//文件系统类加载器
publicclassFileSystemClassLoaderextendsClassLoader{

privateStringrootDir;

publicFileSystemClassLoader(StringrootDir){
this.rootDir=rootDir;
}

//获取类的字节码
@Override
protectedClass<?>findClass(Stringname)throwsClassNotFoundException{
byte[]classData=getClassData(name);//获取类的字节数组
if(classData==null){
thrownewClassNotFoundException();
}else{
returndefineClass(name,classData,0,classData.length);
}
}

privatebyte[]getClassData(StringclassName){
//读取类文件的字节
Stringpath=classNameToPath(className);
try{
InputStreamins=newFileInputStream(path);
ByteArrayOutputStreambaos=newByteArrayOutputStream();
intbufferSize=4096;
byte[]buffer=newbyte[bufferSize];
intbytesNumRead=0;
//读取类文件的字节码
while((bytesNumRead=ins.read(buffer))!=-1){
baos.write(buffer,0,bytesNumRead);
}
returnbaos.toByteArray();
}catch(IOExceptione){
e.printStackTrace();
}
returnnull;
}

privateStringclassNameToPath(StringclassName){
//得到类文件的完全路径
returnrootDir+File.separatorChar
+className.replace('.',File.separatorChar)+".class";
}
}

packagecom.example;

publicclassSample{

privateSampleinstance;

publicvoidsetSample(Objectinstance){
System.out.println(instance.toString());
this.instance=(Sample)instance;
}
}

packageclassloader;

importjava.lang.reflect.Method;

publicclassClassIdentity{

publicstaticvoidmain(String[]args){
newClassIdentity().testClassIdentity();
}

publicvoidtestClassIdentity(){
StringclassDataRootPath="C:\\Users\\JackZhou\\Documents\\NetBeansProjects\\classloader\\build\\classes";
FileSystemClassLoaderfscl1=newFileSystemClassLoader(classDataRootPath);
FileSystemClassLoaderfscl2=newFileSystemClassLoader(classDataRootPath);
StringclassName="com.example.Sample";
try{
Class<?>class1=fscl1.loadClass(className);//加载Sample类
Objectobj1=class1.newInstance();//创建对象
Class<?>class2=fscl2.loadClass(className);
Objectobj2=class2.newInstance();
MethodsetSampleMethod=class1.getMethod("setSample",java.lang.Object.class);
setSampleMethod.invoke(obj1,obj2);
}catch(Exceptione){
e.printStackTrace();
}
}
}

5.2网络类加载器

packageclassloader;

importjava.io.ByteArrayOutputStream;
importjava.io.InputStream;
importjava.net.URL;

publicclassNetworkClassLoaderextendsClassLoader{

privateStringrootUrl;

publicNetworkClassLoader(StringrootUrl){
//指定URL
this.rootUrl=rootUrl;
}

//获取类的字节码
@Override
protectedClass<?>findClass(Stringname)throwsClassNotFoundException{
byte[]classData=getClassData(name);
if(classData==null){
thrownewClassNotFoundException();
}else{
returndefineClass(name,classData,0,classData.length);
}
}

privatebyte[]getClassData(StringclassName){
//从网络上读取的类的字节
Stringpath=classNameToPath(className);
try{
URLurl=newURL(path);
InputStreamins=url.openStream();
ByteArrayOutputStreambaos=newByteArrayOutputStream();
intbufferSize=4096;
byte[]buffer=newbyte[bufferSize];
intbytesNumRead=0;
//读取类文件的字节
while((bytesNumRead=ins.read(buffer))!=-1){
baos.write(buffer,0,bytesNumRead);
}
returnbaos.toByteArray();
}catch(Exceptione){
e.printStackTrace();
}
returnnull;
}

privateStringclassNameToPath(StringclassName){
//得到类文件的URL
returnrootUrl+"/"
+className.replace('.','/')+".class";
}
}

packageclassloader;

publicinterfaceVersioned{

StringgetVersion();
}

packageclassloader;

publicinterfaceICalculatorextendsVersioned{

Stringcalculate(Stringexpression);
}

packagecom.example;

importclassloader.ICalculator;

publicclassCalculatorBasicimplementsICalculator{

@Override
publicStringcalculate(Stringexpression){
returnexpression;
}

@Override
publicStringgetVersion(){
return"1.0";
}
}

packagecom.example;

importclassloader.ICalculator;

publicclassCalculatorAdvancedimplementsICalculator{

@Override
publicStringcalculate(Stringexpression){
return"Resultis"+expression;
}

@Override
publicStringgetVersion(){
return"2.0";
}
}

packagecom.example;

importclassloader.ICalculator;

publicclassCalculatorAdvancedimplementsICalculator{

@Override
publicStringcalculate(Stringexpression){
return"Resultis"+expression;
}

@Override
publicStringgetVersion(){
return"2.0";
}
}

packageclassloader;

publicclassCalculatorTest{

publicstaticvoidmain(String[]args){
Stringurl="http://localhost:8080/ClassloaderTest/classes";
NetworkClassLoaderncl=newNetworkClassLoader(url);
StringbasicClassName="com.example.CalculatorBasic";
StringadvancedClassName="com.example.CalculatorAdvanced";
try{
Class<?>clazz=ncl.loadClass(basicClassName);//加载一个版本的类
ICalculatorcalculator=(ICalculator)clazz.newInstance();//创建对象
System.out.println(calculator.getVersion());
clazz=ncl.loadClass(advancedClassName);//加载另一个版本的类
calculator=(ICalculator)clazz.newInstance();
System.out.println(calculator.getVersion());
}catch(Exceptione){
e.printStackTrace();
}
}

}