JVM之双亲委派源码解读

loadClass（）方法

，双亲委派模型的保证，破坏双亲委派模型重写loadClass方法

    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            //检查是否被加载过
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                	//如果父加载器不是空，调用父加载器的loadclass
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                    //父加载器为空，查询是否被根加载器加载过，没有返回空，抛出异常
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                   
                    // from the non-null parent class loader
                    //如果父加载器没找到，抛出ClassNotFound异常
                }
				//如果父加载器找不到，调用本身的findClass方法加载
                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();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

URLClassLoader的findclass方法

自定义ClassLoader,重写findClass方法

/**
     * Finds and loads the class with the specified name from the URL search
     * path. Any URLs referring to JAR files are loaded and opened as needed
     * until the class is found.
     *
     * @param name the name of the class
     * @return the resulting class
     * @exception ClassNotFoundException if the class could not be found,
     *            or if the loader is closed.
     * @exception NullPointerException if {@code name} is {@code null}.
     * 在指定的URL路径寻找class
     */
    protected Class<?> findClass(final String name)
        throws ClassNotFoundException
    {
        final Class<?> result;
        try {
            result = AccessController.doPrivileged(
                new PrivilegedExceptionAction<Class<?>>() {
                    public Class<?> run() throws ClassNotFoundException {
                        String path = name.replace('.', '/').concat(".class");
                        Resource res = ucp.getResource(path, false);
                        if (res != null) {
                            try {
                                return defineClass(name, res);
                            } catch (IOException e) {
                                throw new ClassNotFoundException(name, e);
                            }
                        } else {
                            return null;
                        }
                    }
                }, acc);
        } catch (java.security.PrivilegedActionException pae) {
            throw (ClassNotFoundException) pae.getException();
        }
        if (result == null) {
            throw new ClassNotFoundException(name);
        }
        return result;
    }

自定义classLoader

public class MyClassLoader extends ClassLoader {

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        File f = new File("c:/test/", name.replace(".", "/").concat(".class"));
        FileInputStream fis = null;
        ByteArrayOutputStream baos = null;
        try {
            fis = new FileInputStream(f);
            baos = new ByteArrayOutputStream();
            int b = 0;

            while ((b = fis.read()) != 0) {
                baos.write(b);
            }

            byte[] bytes = baos.toByteArray();

			//defineClass 根据字节数组定义Class，跟进去时native方法
            return defineClass(name, bytes, 0, bytes.length);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if(baos!=null){
                try {
                    baos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(fis!=null){
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

        }
        return super.findClass(name); //throws ClassNotFoundException
    }