ClassLoader JVM类加载器

最新推荐文章于 2025-09-23 16:40:33 发布

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本文详细介绍了Java中的类加载机制，包括四种类型的类加载器：引导类加载器、扩展类加载器、系统类加载器及自定义类加载器。通过实例代码展示了不同类加载器如何加载各自的类路径。

ClassLoader类加载器：4个
1.bootstrap classloader引导类加载器。加载jdk1.8.0_111/jre/lib下的所有的jar包。没有parent加载器(引导类加载器是c++实现的)
2.extension classloader扩展类加载器。加载jdk1.8.0_111/jre/lib/ext下面的所有jar包。parent is引导类加载器
3.system classloader系统类加载器。加载所有依赖的jar包(如maven下面的包)，classes和 test-classes下所有的类。parent is扩展类加载器

4.自定义类加载器。 parent is系统类加载器

public static void main(){

//获取引导类加载器加载的jar路径。getBootstrapClassPath获取加载的类路径
URL[] urls = sun.misc.Launcher.getBootstrapClassPath().getURLs();
for(URL url : urls){
System.our.println(url.getPath());
}
//扩展类加载器加载的jar路径
URLClassLoader extClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader().getParent();
urls = extClassLoader.getURLs();
for(URL url : urls){
System.our.println(url.getPath());
}
//系统类加载器加载的jar路径
URLClassLoader appClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader();
urls = appClassLoader.getURLs();
for(URL url : urls){
System.our.println(url.getPath());
}

}

实现一个自定义加载器：

//自定义类加载器继承ClassLoader,将ClassLoader里面的loadClass方法拷贝出来从新写
public class MyClassLoader extends ClassLoader{
public MyClassLoader(){}
String path;
public MyClassLoader(String path){
this.path = path;
}
@Override //类加载器的入口方法loadClass
public Class<?> loadClass(String name) throws ClassNotFoundException{
//同步快，主要给并行类加载器，提高性能
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) { //假如类已经被加载直接返回
long t0 = System.nanoTime();
//委派模型。
c = super.loadClass(name);
/* try {
//委派 parent先去加载 the class
if (parent != null) {
c = parent.loadClass(name, false);
} else {
//没有parent加载器则用此加载。引导类加载器没有parent（使用c++写的）。
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}*/
//如果parent没有加载进来 the class，则自己加载 the class，用findClass
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
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();
}
}
//class校验的解析
/* if (resolve) {
resolveClass(c);
}*/
return c;
}
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
FileInputStream fis = null;
path = path + name.replace(".","\") + ".class";
try{
fis = new FileInputStream(path);
int len = fis.available();//字节数组的长度
byte[] b = new byte[len];
//根据文件流读取文件字节数组
fis.read(b, 0, len);
return defineClass(name, b, 0 ,len);
}catch(Exception e){
e.print
}finally{
if(fis != null){
try{
fis.close();
}catch(IOException e){
e.printStackTrace();
}
}
}
return super.findClass(name);
}
}
public class ClassLoaderTest{
public void testLoader(){
System.out.println("is loader ok");
}
}
public class MyClassLoaderTest{
public static void main(){
String path = "d:\\com\\it\\classes\\";
MyClassLoader myClassLoader new MyClassLoader(path);
try{
Class c = myClassLoader.loadClass("com.it.classload.ClassLoaderTest");//加载自定义类
System.out.println(c.getName());
}catch(ClassNotFoundException e){
e.printStrackTrace();
}
}
}
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