Java ClassLoader深入研究（二）

五、关于两个ClassLoader载入同一个类

注意：由两个不同的ClassLoader载入的同一个类，其是不同类型的，因此如果进行赋值会报ClassCastException

示例：

 

package Test;
import java.io.*;
import java.util.*;
public class TestClassLoader extends ClassLoader {

    protected Map<String, Class> cache = new HashMap<String, Class>();

    public TestClassLoader() {
        super();
    }

    protected Class<?> findClass(String name) throws ClassNotFoundException {
        if (cache.get(name) != null) {
            return cache.get(name);
        }
        try {
            String tname = name.replace('.', '/');
            File file = new File("c://" + tname + ".class");
            FileInputStream in = new FileInputStream(file);
            BufferedInputStream bufIn = new BufferedInputStream(in);
            ByteArrayOutputStream byteOut = new ByteArrayOutputStream();
            BufferedOutputStream bufOut = new BufferedOutputStream(byteOut);
            byte[] buffer = new byte[4096];
            int len = -1;
            while((len = bufIn.read(buffer)) != -1) {
                bufOut.write(buffer, 0, len);
            }
            bufOut.flush();
            byteOut.flush();
            byte[] data = byteOut.toByteArray();
            Class cls = defineClass(name, data, 0, data.length);
            cache.put(name, cls);
            return cls;
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }
}
class TestLoaderA {
    public static void main(String[] args) throws InstantiationException, IllegalAccessException, ClassNotFoundException {
        TestClassLoader loader = new TestClassLoader();
        System.out.println(TestAImp1.class.getClassLoader());
        System.out.println(loader.findClass("Test.TestAImp1").getClassLoader());
        TestAImp1 test = (TestAImp1)loader.findClass("Test.TestAImp1").newInstance();
        test.test();

    }
}
public class TestAImp1 extends AbstractTestA {
    public void test() {
        System.out.println(Thread.currentThread().toString());
        System.out.println("Test");
    }
}

package Test;
import java.io.*;
import java.util.*;
public class TestClassLoader extends ClassLoader {
	
	protected Map<String, Class> cache = new HashMap<String, Class>();
	
	public TestClassLoader() {
		super();
	}
	
	protected Class<?> findClass(String name) throws ClassNotFoundException {
		if (cache.get(name) != null) {
			return cache.get(name);
		}
		try {
			String tname = name.replace('.', '/');
			File file = new File("c://" + tname + ".class");
			FileInputStream in = new FileInputStream(file);
			BufferedInputStream bufIn = new BufferedInputStream(in);
			ByteArrayOutputStream byteOut = new ByteArrayOutputStream();
			BufferedOutputStream bufOut = new BufferedOutputStream(byteOut);
			byte[] buffer = new byte[4096];
			int len = -1;
			while((len = bufIn.read(buffer)) != -1) {
				bufOut.write(buffer, 0, len);
			}
			bufOut.flush();
			byteOut.flush();
			byte[] data = byteOut.toByteArray();
			Class cls = defineClass(name, data, 0, data.length);
			cache.put(name, cls);
			return cls;
		} catch (IOException e) {
			e.printStackTrace();
		}
		return null;
	}
}
class TestLoaderA {
	public static void main(String[] args) throws InstantiationException, IllegalAccessException, ClassNotFoundException {
		TestClassLoader loader = new TestClassLoader();
		System.out.println(TestAImp1.class.getClassLoader());
		System.out.println(loader.findClass("Test.TestAImp1").getClassLoader());
		TestAImp1 test = (TestAImp1)loader.findClass("Test.TestAImp1").newInstance();
		test.test();
		
	}
}
public class TestAImp1 extends AbstractTestA {
	public void test() {
		System.out.println(Thread.currentThread().toString());
		System.out.println("Test");
	}
}

 

 运行以上代码结果：

sun.misc.Launcher$AppClassLoader@197d257
Test.TestClassLoader@1b90b39
Exception in thread "main" java.lang.ClassCastException: Test.TestAImp1
 at Test.TestLoaderA.main(TestClassLoader.java:84)