JDK的动态代理为什么需要接口

发表于 | 分类于
字数统计: 2,910 | 阅读时长 ≈ 16

JDK的动态代理为什么需要接口

JDK动态代理实现方式

实现步骤：

  1. 创建一个接口 ProxyInterface

    1
    2
    3
    public interface ProxyInterface {
        public void printSomething();
    }

  2. 编写他得简单实现类 ProxyImpl

1
2
3
4
5
6
public class ProxyImpl implements ProxyInterface {
    @Override
    public void printSomething() {
        System.out.println("反向代理成功！");
    }
}

  1. 编写调用处理程序InvocationHandler，实现其invoke方法，后面具体反向代理，会进入这里面按照下面invoke里面得顺序执行。三个参数，分别代表传入得代理实现类，此次调用得方法名称，有关参数。这是平时使用得动态代理得核心方法。

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    public class ProxyInvocationHandler implements InvocationHandler {

        private Object obj;

        public void setObj(Object obj) {
            this.obj = obj;
        }

        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            System.out.println("代理前！");
            System.out.println("proxy =" + proxy.getClass().toString());
            System.out.println("method = " + method.getName());
            System.out.println("args = " + args);
            Object obj = method.invoke(this.obj,args);
            System.out.println("代理后");
            //注意 如果被代理的方法如果有返回值,这里也需要返回,否则可能会报 NullPointerException
            return obj;
        }

    }

    1. 编写调用类

      1
      2
      3
      4
      5
      6
      7
      8
      9
      10
      public class ProxyMain {
          public static void main(String[] args) {
              ProxyInvocationHandler demo = new ProxyInvocationHandler();
              demo.setObj(new ProxyImpl());
              ProxyInterface o = (ProxyInterface)Proxy.newProxyInstance(ProxyMain.class.getClassLoader(),
                      new Class[]{ProxyInterface.class}, demo);
              o.printSomething();

          }
      }

      启动后，控制台输出

      1
      2
      3
      4
      5
      6
      代理前！
      proxy =class com.sun.proxy.$Proxy0
      method = printSomething
      args = null
      反向代理成功！
      代理后

为什么JDK动态代理需要接口

主要的秘密在newProxyInstance这个方法里

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
@CallerSensitive
    public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);

        final Class<?>[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        /*
         * Look up or generate the designated proxy class.
         */
        //循环生成指定的代理类
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }

            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction<Void>() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            return cons.newInstance(new Object[]{h});
        } catch (IllegalAccessException|InstantiationException e) {
            throw new InternalError(e.toString(), e);
        } catch (InvocationTargetException e) {
            Throwable t = e.getCause();
            if (t instanceof RuntimeException) {
                throw (RuntimeException) t;
            } else {
                throw new InternalError(t.toString(), t);
            }
        } catch (NoSuchMethodException e) {
            throw new InternalError(e.toString(), e);
        }
    }

调用getProxyClass()

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
/**
     * Generate a proxy class.  Must call the checkProxyAccess method
     * to perform permission checks before calling this.
     */
    private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }

        // If the proxy class defined by the given loader implementing
        // the given interfaces exists, this will simply return the cached copy;
      //给定的接口存在，这只会返回缓存的副本
        // otherwise, it will create the proxy class via the ProxyClassFactory
      //否则，它将通过ProxyClassFactory创建代理类
        return proxyClassCache.get(loader, interfaces);
    }

proxyClassCache是Proxy的静态变量，是WeakCache类，里面封装了两个类KeyFactory、ProxyClassFactory，都是BiFunction函数式接口，作转换用。KeyFactory 用与生成Cache中的key，ProxyClassFactory用于生成Cache中的value，也就是代理类。

关注ProxyClassFactory中的apply方法

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
@Override
        public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {

            Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            for (Class<?> intf : interfaces) {
                /*
                 * Verify that the class loader resolves the name of this
                 * interface to the same Class object.
                 */
                Class<?> interfaceClass = null;
                try {
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) {
                }
                if (interfaceClass != intf) {
                    throw new IllegalArgumentException(
                        intf + " is not visible from class loader");
                }
                /*
                 * Verify that the Class object actually represents an
                 * interface.
                 */
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
                /*
                 * Verify that this interface is not a duplicate.
                 */
                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                    throw new IllegalArgumentException(
                        "repeated interface: " + interfaceClass.getName());
                }
            }

            String proxyPkg = null;     // package to define proxy class in
            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;

            /*
             * Record the package of a non-public proxy interface so that the
             * proxy class will be defined in the same package.  Verify that
             * all non-public proxy interfaces are in the same package.
             */
            for (Class<?> intf : interfaces) {
                int flags = intf.getModifiers();
                if (!Modifier.isPublic(flags)) {
                    accessFlags = Modifier.FINAL;
                    String name = intf.getName();
                    int n = name.lastIndexOf('.');
                    String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
                    if (proxyPkg == null) {
                        proxyPkg = pkg;
                    } else if (!pkg.equals(proxyPkg)) {
                        throw new IllegalArgumentException(
                            "non-public interfaces from different packages");
                    }
                }
            }

            if (proxyPkg == null) {
                // if no non-public proxy interfaces, use com.sun.proxy package
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            /*
             * Choose a name for the proxy class to generate.
             */
            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * Generate the specified proxy class.
             关注这里，这里生成了增强的Bean字节码文件
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try {
              //调用native方法加载到内存
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {
                /*
                 * A ClassFormatError here means that (barring bugs in the
                 * proxy class generation code) there was some other
                 * invalid aspect of the arguments supplied to the proxy
                 * class creation (such as virtual machine limitations
                 * exceeded).
                 */
                throw new IllegalArgumentException(e.toString());
            }
        }
    }

可以看出内部调用了ProxyGenerator的generateProxyClass方法。然后再调用defineClass0方发

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
		//创建一个对象,用来生成代理类的字节码
        ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
  		//生成代理类字节码
        final byte[] var4 = var3.generateClassFile();
        if (saveGeneratedFiles) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    try {
                        int var1 = var0.lastIndexOf(46);
                        Path var2;
                        if (var1 > 0) {
                            Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar));
                            Files.createDirectories(var3);
                            var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                        } else {
                            var2 = Paths.get(var0 + ".class");
                        }
					// 简直解码写入文件
                        Files.write(var2, var4, new OpenOption[0]);
                        return null;
                    } catch (IOException var4x) {
                        throw new InternalError("I/O exception saving generated file: " + var4x);
                    }
                }
            });
        }

        return var4;
    }

然后我们再来看下生成字节码的方法

代码比较长,而且东西比较多.如果不懂字节码的话,很难看懂.

这里简单的提一下class文件字节码结构

class 文件由下面十个部分组成

  • 魔数（Magic Number）
  • 版本号（Minor&Major Version）
  • 常量池（Constant Pool）
  • 类访问标记(Access Flags)
  • 类索引（This Class）
  • 超类索引（Super Class）
  • 接口表索引（Interfaces）
  • 字段表（Fields）
  • 方法表（Methods）
  • 属性表（Attributes）

然后我们找到给超类索引设置值的地方.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
private byte[] generateClassFile() {
        this.addProxyMethod(hashCodeMethod, Object.class);
        this.addProxyMethod(equalsMethod, Object.class);
        this.addProxyMethod(toStringMethod, Object.class);
        Class[] var1 = this.interfaces;
        int var2 = var1.length;

        int var3;
        Class var4;
        for(var3 = 0; var3 < var2; ++var3) {
            var4 = var1[var3];
            Method[] var5 = var4.getMethods();
            int var6 = var5.length;

            for(int var7 = 0; var7 < var6; ++var7) {
                Method var8 = var5[var7];
                this.addProxyMethod(var8, var4);
            }
        }

        Iterator var11 = this.proxyMethods.values().iterator();

        List var12;
        while(var11.hasNext()) {
            var12 = (List)var11.next();
            checkReturnTypes(var12);
        }

        Iterator var15;
        try {
            this.methods.add(this.generateConstructor());
            var11 = this.proxyMethods.values().iterator();

            while(var11.hasNext()) {
                var12 = (List)var11.next();
                var15 = var12.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.ProxyMethod var16 = (ProxyGenerator.ProxyMethod)var15.next();
                    this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
                    this.methods.add(var16.generateMethod());
                }
            }

            this.methods.add(this.generateStaticInitializer());
        } catch (IOException var10) {
            throw new InternalError("unexpected I/O Exception", var10);
        }

        if (this.methods.size() > 65535) {
            throw new IllegalArgumentException("method limit exceeded");
        } else if (this.fields.size() > 65535) {
            throw new IllegalArgumentException("field limit exceeded");
        } else {
            this.cp.getClass(dotToSlash(this.className));
            this.cp.getClass("java/lang/reflect/Proxy");
            var1 = this.interfaces;
            var2 = var1.length;

            for(var3 = 0; var3 < var2; ++var3) {
                var4 = var1[var3];
                this.cp.getClass(dotToSlash(var4.getName()));
            }

            this.cp.setReadOnly();
            ByteArrayOutputStream var13 = new ByteArrayOutputStream();
            DataOutputStream var14 = new DataOutputStream(var13);

            try {
                var14.writeInt(-889275714); //cafebabe
                var14.writeShort(0); // 小版本号
                var14.writeShort(49); // 大版本号
                this.cp.write(var14); // 常量池
              
                var14.writeShort(this.accessFlags); //类访问标示
                var14.writeShort(this.cp.getClass(dotToSlash(this.className))); //当前类
                var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy")); //父类
                var14.writeShort(this.interfaces.length); //实现的接口
                Class[] var17 = this.interfaces;
                int var18 = var17.length;

                for(int var19 = 0; var19 < var18; ++var19) {
                    Class var22 = var17[var19];
                    var14.writeShort(this.cp.getClass(dotToSlash(var22.getName())));
                }

                var14.writeShort(this.fields.size());
                var15 = this.fields.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next();
                    var20.write(var14);
                }

                var14.writeShort(this.methods.size());
                var15 = this.methods.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
                    var21.write(var14);
                }

                var14.writeShort(0);
                return var13.toByteArray();
            } catch (IOException var9) {
                throw new InternalError("unexpected I/O Exception", var9);
            }
        }
    }

从上面的字节码可以看出,生成的代理类继承了java/lang/reflect/Proxy.

我们再测试下生成的文件内容

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
package dynamic;

import sun.misc.ProxyGenerator;

import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

interface Dao {
    void save();
}

class DaoImpl implements Dao {
    @Override
    public void save() {
        System.out.println("save...");
    }
}
public class DynamicProxyTest {

    /**
     * 保存 JDK 动态代理生产的类
     * @param filePath 保存路径，默认在项目路径下生成 $Proxy0.class 文件
     */
    private static void saveProxyFile(String... filePath) {
        if (filePath.length != 0) {
            System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        } else {
            FileOutputStream out = null;
            try {
                byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", DaoImpl.class.getInterfaces());
                out = new FileOutputStream("src/" + "$Proxy0.class");
                out.write(classFile);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                try {
                    if (out != null) {
                        out.flush();
                        out.close();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    public static void main(String[] args) throws Exception {
        saveProxyFile();

        Object target = new DaoImpl();

        /**
         * loader：业务对象的类加载器
         * interfaces：业务对象实现的所有接口
         * public static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces)
         */
        Class<?> proxyClass = Proxy.getProxyClass(DaoImpl.class.getClassLoader(), DaoImpl.class.getInterfaces());
        InvocationHandler handler = new InvocationHandler() {
            /**
             * @param proxy 代理对象
             * @param method 代理的方法对象
             * @param args 方法调用时参数
             */
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                Object result = null;
                if (method.getName().equals("save")) {
                    System.out.println("before...");
                    result = method.invoke(target, args);
                    System.out.println("after...");
                }
                return result;
            }
        };
        Dao userDao = (Dao) proxyClass.getConstructor(InvocationHandler.class).newInstance(handler);
        userDao.save();
    }
}

看到生成的文件，反编译之后

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

import dynamic.Dao;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $Proxy0 extends Proxy implements Dao {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    private static Method m0;

    public $Proxy0(InvocationHandler var1) throws  {
        super(var1);
    }

    public final boolean equals(Object var1) throws  {
        try {
            return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final void save() throws  {
        try {
        	//这里 就是调用invoke执行的方法
            super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final String toString() throws  {
        try {
            return (String)super.h.invoke(this, m2, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final int hashCode() throws  {
        try {
            return (Integer)super.h.invoke(this, m0, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m3 = Class.forName("dynamic.Dao").getMethod("save");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

在调用save方法时候，会调用 super.h.invoke(this, m3, (Object[])null);由下面的静态代码块可知，m3是我们接口实现的原生方法，而h就是我们实现的InvocationHandler

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
public class Proxy implements java.io.Serializable {

    private static final long serialVersionUID = -2222568056686623797L;

    /** parameter types of a proxy class constructor */
    private static final Class<?>[] constructorParams =
        { InvocationHandler.class };

    /**
     * a cache of proxy classes
     */
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

    /**
     * the invocation handler for this proxy instance.
     * @serial
     */
    protected InvocationHandler h;

个人理解

因为生成的代理类集成了Proxy，我们知道java是单继承的，如果 原代理类是实现类，不能保证没有它没有继承其他类，就会出现错误，为了防止这种错误，所以就只允许代理接口。

0%