URLClassLoader之ucp

Topic： 從ClassLoader源碼中學習URLClassLoader

首先 說明一下ClassLoader的核心 – loadClass

    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        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.
                    c = findClass(name);
                }
            }
            if (resolve) { resolveClass(c); }
            return c;
        }
    }

    // ClassLoader method
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
    }

其中findClass是關鍵：一來沒有具體實現，二來修飾符是protected，所以 很明顯是爲了“開閉”。

接着，查看URLClassLoader的findClass實現

// URLClassLoader overwrite
    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;
    }

如果這塊的核心邏輯，那必然是 – ucp.getResource

從上可知，所有繼承至URLClassLoader的子類（不overwrite findClass的情況下），都是 只能通過ucp來進行資源的加載。所以ucp很重要，它指定了資源的來源，同時也限制了資源的來源。

• 指定資源的來源：可以來自於Local，來自於Remote；可以是Jar，也可以是War等等。
• 限制資源的來源：當指定了該ucp，那麼該ClassLoader的資源來源也就被限制只能是來自於這裏。

然後，查看ExtClassLoader & AppClassLoader的繼承關係

很明顯。那麼接下來，查看一下它們的ucp

• ExtClassLoader

    static class ExtClassLoader extends URLClassLoader {
        public static Launcher.ExtClassLoader getExtClassLoader() throws IOException {
            final File[] var0 = getExtDirs();

            try {
                return (Launcher.ExtClassLoader)AccessController.doPrivileged(new PrivilegedExceptionAction<Launcher.ExtClassLoader>() {
                    public Launcher.ExtClassLoader run() throws IOException {
                        int var1 = var0.length;

                        for(int var2 = 0; var2 < var1; ++var2) {
                            MetaIndex.registerDirectory(var0[var2]);
                        }

                        return new Launcher.ExtClassLoader(var0);
                    }
                });
            } catch (PrivilegedActionException var2) {
                throw (IOException)var2.getException();
            }
        }

        public ExtClassLoader(File[] var1) throws IOException {
            super(getExtURLs(var1), (ClassLoader)null, Launcher.factory);
            SharedSecrets.getJavaNetAccess().getURLClassPath(this).initLookupCache(this);
        }

        private static File[] getExtDirs() {
            String var0 = System.getProperty("java.ext.dirs");
            File[] var1;
            if (var0 != null) {
                StringTokenizer var2 = new StringTokenizer(var0, File.pathSeparator);
                int var3 = var2.countTokens();
                var1 = new File[var3];

                for(int var4 = 0; var4 < var3; ++var4) {
                    var1[var4] = new File(var2.nextToken());
                }
            } else {
                var1 = new File[0];
            }

            return var1;
        }

        private static URL[] getExtURLs(File[] var0) throws IOException {
            Vector var1 = new Vector();

            for(int var2 = 0; var2 < var0.length; ++var2) {
                String[] var3 = var0[var2].list();
                if (var3 != null) {
                    for(int var4 = 0; var4 < var3.length; ++var4) {
                        if (!var3[var4].equals("meta-index")) {
                            File var5 = new File(var0[var2], var3[var4]);
                            var1.add(Launcher.getFileURL(var5));
                        }
                    }
                }
            }

            URL[] var6 = new URL[var1.size()];
            var1.copyInto(var6);
            return var6;
        }
    }

結論：通過getExtClassLoader來獲取ExtClassLoader時，直接採用環境變量中的“java.ext.dirs”作爲資源

• AppClassLoader

    static class AppClassLoader extends URLClassLoader {
        final URLClassPath ucp = SharedSecrets.getJavaNetAccess().getURLClassPath(this);

        public static ClassLoader getAppClassLoader(final ClassLoader var0) throws IOException {
            final String var1 = System.getProperty("java.class.path");
            final File[] var2 = var1 == null ? new File[0] : Launcher.getClassPath(var1);
            return (ClassLoader)AccessController.doPrivileged(new PrivilegedAction<Launcher.AppClassLoader>() {
                public Launcher.AppClassLoader run() {
                    URL[] var1x = var1 == null ? new URL[0] : Launcher.pathToURLs(var2);
                    return new Launcher.AppClassLoader(var1x, var0);
                }
            });
        }

        AppClassLoader(URL[] var1, ClassLoader var2) {
            super(var1, var2, Launcher.factory);
            this.ucp.initLookupCache(this);
        }
    }

結論：通過getAppClassLoader來獲取AppClassLoader時，直接採用環境變量中的“java.class.path”作爲資源

總結

如果想自定義ClassLoader，以實現擴展“資源”，至少需要繼承URLClassLoader並提供期望的ucp。
如果想自定義ClassLoader，並控制雙親委派，則需要繼承URLClassLoader並overwrite findClass邏輯。