1.bootstrap classloader引導類加載器。加載jdk1.8.0_111/jre/lib下的 所有的jar包。沒有parent加載器(引導類加載器是c++實現的)
2.extension classloader擴展類加載器。加載jdk1.8.0_111/jre/lib/ext下面的所有jar包。parent is引導類加載器
3.system classloader系統類加載器。加載 所有依賴的jar包(如maven下面的包)，classes和 test-classes下所有的類。parent is擴展類加載器
4.自定義類加載器。 parent is系統類加載器
public static void main(){
//獲取引導類加載器 加載的jar路徑。getBootstrapClassPath獲取加載的類路徑URL[] urls = sun.misc.Launcher.getBootstrapClassPath().getURLs();
for(URL url : urls){
System.our.println(url.getPath());
}
//擴展類加載器 加載的jar路徑
URLClassLoader extClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader().getParent();
urls = extClassLoader.getURLs();
for(URL url : urls){
System.our.println(url.getPath());
}
//系統類加載器 加載的jar路徑
URLClassLoader appClassLoader = (URLClassLoader)ClassLoader.getSystemClassLoader();
urls = appClassLoader.getURLs();
for(URL url : urls){
System.our.println(url.getPath());
}
}
實現一個自定義加載器：
public class MyClassLoader extends ClassLoader{
public MyClassLoader(){}
String path;
public MyClassLoader(String path){
this.path = path;
}
@Override //類加載器的入口方法loadClass
public Class<?> loadClass(String name) throws ClassNotFoundException{
//同步快，主要給並行類加載器，提高性能
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) { //假如類已經被加載 直接返回
long t0 = System.nanoTime();
//委派模型。
c = super.loadClass(name);
/* try {
//委派 parent先去加載 the class
if (parent != null) {
c = parent.loadClass(name, false);
} else {
//沒有parent加載器則用此加載。引導類加載器沒有parent（使用c++寫的）。
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}*/
//如果parent沒有加載進來 the class，則自己加載 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校驗的解析
/* 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();//字節數組的長度
byte[] b = new byte[len];
//根據文件流 讀取文件字節數組
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");//加載 自定義類
System.out.println(c.getName());
}catch(ClassNotFoundException e){
e.printStrackTrace();
}
}
}
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