第九篇 JVM核心机制之JVM运行和类加载全过程（四）

（1）从上面源码看出，调用loadClass时会先根据委派模型在父加载器中加载，如果加载失败，则会调用当前加载器的findClass来完成加载。

（2）因此我们自定义的类加载器只需要继承ClassLoader，并覆盖findClass方法，下面是一个实际例子，在该例中我们用自定义的类加载器去加载我们事先准备好的class文件。

  

/**
 * 自定义文件系统类加载器
 *
 * @author Zhang XiaoDao
 *
 */
public class FileSystemClassLoader extends ClassLoader{
    
    private String rootDir;

    public FileSystemClassLoader(String rootDir) {
        super();
        this.rootDir = rootDir;
    }
    
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        Class<?> c = findLoadedClass(name);
        //应该要先查询有没有加载过这个类，如果已经加载，则直接返回这个类。如果没有加载，则返回新的类
        if(c != null){
            return c;
        }else{
            ClassLoader parent = this.getParent();
            c = parent.loadClass(name);
            if(c != null){
                return c;
            }else{
                byte[] classData = getClassData(name);
                if(classData == null){
                    throw new ClassNotFoundException();
                }else{
                    c = defineClass(name, classData, 0, classData.length);
                }
            }
        }
        return c;
    }
    
    private byte[] getClassData(String classname){//com.bjsxt.test.User  --> d:/java/ com/bjsxt/test/User.class
        String path = rootDir + "/" + classname.replace('.', '/') + ".class";
        //IOUtils 可以使用它将流中的数据转成字节数组
        InputStream is = null;
        ByteArrayOutputStream  baOutputStream = new ByteArrayOutputStream();
        try {
            is = new FileInputStream(path);
            byte[] buffer = new byte[1024];
            int temp = 0;
            while((temp= is.read(buffer))!=-1){
                baOutputStream.write(buffer, 0, temp);
            }
            return baOutputStream.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }finally {
            if(is != null){
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(baOutputStream != null){
                try {
                    baOutputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }        
//        return null;        
    }
    
}

 

/**
 * 
 * 测试自定义加载器
 *
 * @author Zhang XiaoDao
 *
 */
public class Demo03 {
    public static void main(String[] args) throws ClassNotFoundException {
        FileSystemClassLoader loader = new FileSystemClassLoader("d:/java");
        FileSystemClassLoader loader2 = new FileSystemClassLoader("d:/java");
        
        Class<?> c1 = loader.loadClass("com.zzp.user");
        Class<?> c2 = loader.loadClass("com.zzp.user");
        //同一个类被不同的加载器加载，jvm会认为是不同的类
        Class<?> c3 = loader2.loadClass("com.zzp.user");
        Class<?> c4 = loader2.loadClass("java.lang.String");
        Class<?> c5 = loader2.loadClass("com.Demo01");
        
        
        System.out.println(c1);
        System.out.println(c2);
        System.out.println(c3.getClassLoader());//自定义类加载器
        System.out.println(c4.getClassLoader());//引导类加载器
        System.out.println(c5.getClassLoader());//系统类加载器
    }
}

网络类加载器：

/**
 * 网络类加载器
 *
 * @author Zhang XiaoDao
 *
 */
public class NetClassLoader extends ClassLoader{
    
    private String rootUrl;

    public NetClassLoader(String rootUrl) {
        super();
        this.rootUrl = rootUrl;
    }
    
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        Class<?> c = findLoadedClass(name);
        //应该要先查询有没有加载过这个类，如果已经加载，则直接返回这个类。如果没有加载，则返回新的类
        if(c != null){
            return c;
        }else{
            ClassLoader parent = this.getParent();
            c = parent.loadClass(name);
            if(c != null){
                return c;
            }else{
                byte[] classData = getClassData(name);
                if(classData == null){
                    throw new ClassNotFoundException();
                }else{
                    c = defineClass(name, classData, 0, classData.length);
                }
            }
        }
        return c;
    }
    
    private byte[] getClassData(String classname){//com.bjsxt.test.User  --> d:/java/ com/bjsxt/test/User.class
        String path = rootUrl + "/" + classname.replace('.', '/') + ".class";
        //IOUtils 可以使用它将流中的数据转成字节数组
        InputStream is = null;
        ByteArrayOutputStream  baOutputStream = new ByteArrayOutputStream();
        try {
            URL url = new URL(path);
            is = url.openStream();
            byte[] buffer = new byte[1024];
            int temp = 0;
            while((temp= is.read(buffer))!=-1){
                baOutputStream.write(buffer, 0, temp);
            }
            return baOutputStream.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }finally {
            if(is != null){
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(baOutputStream != null){
                try {
                    baOutputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }        
//        return null;        
    }
    
}