双亲委派模型下,在父类加载器无法加载的情况下再由当前类加载器去加载。具体的实现逻辑在java.util.ClassLoader抽象类的loadClass方法中。在该方法中,先检查是否已经加载过,如果没有,就让父类加载器加载。如果父类加载器服务加载,就调用findClass方法加载。findClass方法是空的,需要用户重载它。所以,按照这样的逻辑,我们在使用loadClass之前需要重载findClass方法。
Boostrap ClassLoader
|
Extension ClassLoader
|
Application ClassLoader
| |
OtherClassLoader1 OtherClassLoader2 ...
public Class loadClass(String name) throws ClassNotFoundException {
return loadClass(name, false);
}
protected Class loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// 1. 检查是否已经加载过
Class c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
if (parent != null) {
//当前加载器存在父类加载器,递归调用父类加载器
c = parent.loadClass(name, false);
} else {
//检查该类是否被Bootstrap加载器加载过,有则返回,否则返回null
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
//既然父类加载器无法加载,则就是用用户自己所重载的类加载器
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();
}
}
if (resolve) {
//链接link所加载的类。该方法名起得不好,有误导性
resolveClass(c);
}
return c;
}
}
protected Class findClass(String name) throws ClassNotFoundException {
//如果没有重载,一调用就抛出异常
throw new ClassNotFoundException(name);
}
findClass的返回值是Class类型,而我们可以使用defineClass方法将一个byte[]转换成Class类型。而byte[]数据可以通过文件读取class字节码文件获取。
protected final Class defineClass(String name, byte[] b, int off, int len) throws ClassFormatError {
return defineClass(name, b, off, len, null);
}
首先,我们定义一个待加载的普通Java类:Student.java, 其中package为空,可以在任何路径下编译该文件成class字节码文件。
public class Student {
public void say(){
System.out.println("hello world");
}
}
进入window的cmd命令行窗口,进入Student.java所在目录,使用命令javac Student.java
获得Student.class文件。
javac Test.java
和java Test
来进行测试。mport java.lang.reflect.Method;
import java.io.FileInputStream;
class MyClassLoader extends ClassLoader {
@Override
protected Class findClass(String name) throws ClassNotFoundException {
try {
byte[] data = loadByte();
//将byte[]数据转换成Class类型的对象
return defineClass(name, data, 0, data.length);
} catch (Exception e) {
e.printStackTrace();
throw new ClassNotFoundException();
}
}
//从硬盘中读取class字节码文件,并转换成byte[]数据
private byte[] loadByte() throws Exception {
String name = "C:/Users/USER/Desktop/temp/Student.class";
FileInputStream fis = new FileInputStream(name);
int len = fis.available();
byte[] data = new byte[len];
fis.read(data);
fis.close();
return data;
}
};
public class Test {
public static void main(String args[]) throws Exception {
MyClassLoader classLoader = new MyClassLoader();
//加载Student的Class类型对象
Class clazz = classLoader.loadClass("Student");
Object obj = clazz.newInstance();
//使用反射的方式获取和执行say()方法
Method helloMethod = clazz.getDeclaredMethod("say", null);
helloMethod.invoke(obj, null);
}
}
谢谢!