cglib源码分析（一）： 缓存和KEY
cglib是一个java 字节码的生成工具，它是对asm的进一步封装，提供了一系列class generator。研究cglib主要是因为它也提供了动态代理功能，这点和jdk的动态代理类似。
一、 Cache的创建
与jdk动态代理一样，cglib也提供了缓存来提高系统的性能，对于已经生成的类，直接使用而不必重复生成。这里不得不提到一个比较重要的抽象类AbstractClassGenerator，它采用了模版方法的设计模式，protected Object create(Object key) 就是模版方法，它定义了类生成的过程。AbstractClassGenerator只有一个构造函数protected AbstractClassGenerator(Source source)，入参是一个Source类型的对象，Source是AbstractClassGenerator里面的一个静态内部类，Source有两个字段 name用来记录class generator，cache 就是缓存，它和jdk动态代理一样都是用了WeakHashMap，并且类型也是<ClassLoader,<Object,Class>>：
protected static class Source { String name; //class generator的name，eg：如果使用Enhancer来生成增强类，name的值就为 net.sf.cglib.proxy. Enhancer Map cache = new WeakHashMap(); public Source(String name) { this.name = name; } }
每个class generator都必须继承AbstractClassGenerator并且实现 public void generateClass(ClassVisitor v) 方法用来生成所需要的类。每个class generator都有独立的缓存，比如 Enhancer 类中 private static final Source SOURCE = new Source(Enhancer.class.getName()); 在BeanGenerator 中 private static final Source SOURCE = new Source(BeanGenerator.class.getName()); 。
二、 Cache的使用
缓存的使用主要封装在AbstractClassGenerator的模版方法create中，下面是create方法的源码：
synchronized (source) { ClassLoader loader = getClassLoader(); Map cache2 = null; cache2 = (Map)source.cache.get(loader); if (cache2 == null) { cache2 = new HashMap(); cache2.put(NAME_KEY, new HashSet()); source.cache.put(loader, cache2); } else if (useCache) { Reference ref = (Reference)cache2.get(key); gen = (Class) (( ref == null ) ? null : ref.get()); } …… 忽略若干代码 if (useCache) { cache2.put(key, new WeakReference(gen)); } }
生成类的缓存是按照ClassLoader来划分的，这是因为类的区分不仅根据类名还根据装载类的ClassLoader，也就是说同一个类被不同的ClassLoader加载，那么它们也是不同的，关于这部分内容可参考 http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-5.html#jvms-5.3 。每个ClassLoader的缓存中都会有一个NAME_KEY 这个主要是用来对生成的class name进行去重，NAME_KEY 会在生成类命名策略里有进一步的说明。此处还使用useCache 来标记是否使用缓存，这给了用户比较灵活的选择。
三、Key的例子
每个生成类在缓存中都会有一个key与之相对应。对于那些只与单个类相关的生成类，可以采用类名作为key。在动态代理中生成类不仅与目标类相关，还与使用的拦截类（MethodInterceptor），过滤类（CallbackFilter）相关，这样的话就要使用multi-vaules key来标识这个生成类，在cglib中multi-vaules 也是动态生成的，KeyFactory 就是生成multi-vaules的工厂类，它是一个抽象类，也就是说它不能被实例化，但是它提供了一系列的静态工厂方法来生成multi-vaules的工厂类，这里很拗口，下面是cglib源码包中的一个例子：
package samples; import net.sf.cglib.core.KeyFactory; public class KeySample { private interface MyFactory { public Object newInstance(int a, char[] b, String d); } public static void main(String[] args) { MyFactory f = (MyFactory)KeyFactory.create(MyFactory.class); Object key1 = f.newInstance(20, new char[]{ 'a', 'b' }, "hello"); Object key2 = f.newInstance(20, new char[]{ 'a', 'b' }, "hello"); Object key3 = f.newInstance(20, new char[]{ 'a', '_' }, "hello"); System.out.println(key1.equals(key2)); System.out.println(key1.toString()); System.out.println(key2.equals(key3)); } }
运行结果是：
true 20, {a, b}, hello false
为了生成multi-vaules 的工厂类，我们必须提供一个接口来描述multi-vaules的结构（上例中该接口为MyFactory），该接口有且只有一个方法newInstance，该方法的返回值必须为Object，该方法的入参可以是任意的对象，元数据类型 或者是任意维的数组 但是入参不能为空，如果为空就和默认的构造函数相冲突。 在分析KeyFactory之前，我们将上例生成的multi-vaules工厂类进行一下反编译(jd-gui 由于版本的问题无法反编译，因而此处使用javap)：
public class samples.KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e extends net.sf.cglib.core.KeyFactory implements samples.KeySample$MyFactory{ public samples.KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e(); Code: 0: aload_0 1: invokespecial #11; //Method net/sf/cglib/core/KeyFactory."<init>":()V 4: return public java.lang.Object newInstance(int, char[], java.lang.String); Code: 0: new #2; //class samples/KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e 3: dup 4: iload_1 5: aload_2 6: aload_3 7: invokespecial #16; //Method "<init>":(I[CLjava/lang/String;)V 10: areturn public samples.KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e(int, char[], java.lang.String); Code: 0: aload_0 1: invokespecial #11; //Method net/sf/cglib/core/KeyFactory."<init>":()V 4: aload_0 5: dup 6: iload_1 7: putfield #20; //Field FIELD_0:I 10: dup 11: aload_2 12: putfield #24; //Field FIELD_1:[C 15: dup 16: aload_3 17: putfield #28; //Field FIELD_2:Ljava/lang/String; 20: return public int hashCode(); Code: 0: sipush 179 3: aload_0 4: getfield #20; //Field FIELD_0:I 7: swap 8: ldc #31; //int 467063 10: imul 11: swap 12: iadd 13: aload_0 14: getfield #24; //Field FIELD_1:[C 17: dup 18: ifnull 48 21: astore_1 22: iconst_0 23: istore_2 24: goto 39 27: aload_1 28: iload_2 29: caload 30: swap 31: ldc #31; //int 467063 33: imul 34: swap 35: iadd 36: iinc 2, 1 39: iload_2 40: aload_1 41: arraylength 42: if_icmplt 27 45: goto 49 48: pop 49: aload_0 50: getfield #28; //Field FIELD_2:Ljava/lang/String; 53: swap 54: ldc #31; //int 467063 56: imul 57: swap 58: dup 59: ifnull 68 62: invokevirtual #35; //Method java/lang/Object.hashCode:()I 65: goto 70 68: pop 69: iconst_0 70: iadd 71: ireturn public boolean equals(java.lang.Object); Code: 0: aload_1 1: instanceof #2; //class samples/KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e 4: ifeq 133 7: aload_0 8: getfield #20; //Field FIELD_0:I 11: aload_1 12: checkcast #2; //class samples/KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e 15: getfield #20; //Field FIELD_0:I 18: if_icmpne 133 21: aload_0 22: getfield #24; //Field FIELD_1:[C 25: aload_1 26: checkcast #2; //class samples/KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e 29: getfield #24; //Field FIELD_1:[C 32: dup2 33: ifnonnull 43 36: ifnonnull 49 39: pop2 40: goto 93 43: ifnull 49 46: goto 53 49: pop2 50: goto 133 53: dup2 54: arraylength 55: swap 56: arraylength 57: if_icmpeq 64 60: pop2 61: goto 133 64: astore_2 65: astore_3 66: iconst_0 67: istore 4 69: goto 86 72: aload_2 73: iload 4 75: caload 76: aload_3 77: iload 4 79: caload 80: if_icmpne 133 83: iinc 4, 1 86: iload 4 88: aload_2 89: arraylength 90: if_icmplt 72 93: aload_0 94: getfield #28; //Field FIELD_2:Ljava/lang/String; 97: aload_1 98: checkcast #2; //class samples/KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e 101: getfield #28; //Field FIELD_2:Ljava/lang/String; 104: dup2 105: ifnonnull 115 108: ifnonnull 121 111: pop2 112: goto 131 115: ifnull 121 118: goto 125 121: pop2 122: goto 133 125: invokevirtual #39; //Method java/lang/Object.equals:(Ljava/lang/Object;)Z 128: ifeq 133 131: iconst_1 132: ireturn 133: iconst_0 134: ireturn public java.lang.String toString(); Code: 0: new #43; //class java/lang/StringBuffer 3: dup 4: invokespecial #44; //Method java/lang/StringBuffer."<init>":()V 7: aload_0 8: getfield #20; //Field FIELD_0:I 11: invokevirtual #48; //Method java/lang/StringBuffer.append:(I)Ljava/lang/StringBuffer; 14: goto 23 17: pop 18: ldc #50; //String null 20: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 23: ldc #55; //String , 25: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 28: aload_0 29: getfield #24; //Field FIELD_1:[C 32: dup 33: ifnull 96 36: swap 37: ldc #57; //String { 39: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 42: swap 43: astore_1 44: iconst_0 45: istore_2 46: goto 72 49: aload_1 50: iload_2 51: caload 52: invokevirtual #60; //Method java/lang/StringBuffer.append:(C)Ljava/lang/StringBuffer; 55: goto 64 58: pop 59: ldc #50; //String null 61: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 64: ldc #55; //String , 66: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 69: iinc 2, 1 72: iload_2 73: aload_1 74: arraylength 75: if_icmplt 49 78: dup 79: dup 80: invokevirtual #63; //Method java/lang/StringBuffer.length:()I 83: iconst_2 84: isub 85: invokevirtual #67; //Method java/lang/StringBuffer.setLength:(I)V 88: ldc #69; //String } 90: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 93: goto 102 96: pop 97: ldc #50; //String null 99: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 102: ldc #55; //String , 104: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 107: aload_0 108: getfield #28; //Field FIELD_2:Ljava/lang/String; 111: dup 112: ifnull 124 115: invokevirtual #71; //Method java/lang/Object.toString:()Ljava/lang/String; 118: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 121: goto 130 124: pop 125: ldc #50; //String null 127: invokevirtual #53; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 130: invokevirtual #72; //Method java/lang/StringBuffer.toString:()Ljava/lang/String; 133: areturn }
从反编译的结果我们可以看出，生成的工厂类为samples.KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e，它继承了net.sf.cglib.core.KeyFactory 类，实现了samples.KeySample$MyFactory接口，同时也实现了工厂方法newInstance，所以上例中key1，key2，key3都是通过该工厂方法产生了key的对象。samples.KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e 中有两个构造函数，一个是默认的构造函数(由于工厂方法newInstance 为非静态方法，所以需要使用默认构造函数来生成第一个对象)，另外一个构造函数的入参和 newInstance 入参一样，newInstance使用有参构造函数来生成multi-values key 对象。samples.KeySample$MyFactory$$KeyFactoryByCGLIB$$7116a61e 为newInstance的每一个入参都生成了一个相同类型的field用来存储key的value。除此之外还提供了三个方法：hashCode, equals, toString.
hashCode 和 equals 在HashMap 的get方法中用于和存储的key进行比较，所以这两个方法是比较重要的。
四、KeyFactory 源码分析
KeyFactory的源码还是比较多的，接下来只对其中的关键代码进行分析：
public void generateClass(ClassVisitor v) { ClassEmitter ce = new ClassEmitter(v); //对定义key工厂类结构的接口进行判断，判断该接口是否只有newInstance一个方法，newInstance的返回值是否为Object Method newInstance = ReflectUtils.findNewInstance(keyInterface); if (!newInstance.getReturnType().equals(Object.class)) { throw new IllegalArgumentException("newInstance method must return Object"); } //获取newInstance的入参类型，此处使用ASM的Type来定义 Type[] parameterTypes = TypeUtils.getTypes(newInstance.getParameterTypes()); ce.begin_class(Constants.V1_2, Constants.ACC_PUBLIC, getClassName(), KEY_FACTORY, new Type[]{ Type.getType(keyInterface) }, Constants.SOURCE_FILE); //生成默认构造函数 EmitUtils.null_constructor(ce); //生成newInstance 工厂方法 EmitUtils.factory_method(ce, ReflectUtils.getSignature(newInstance)); //生成有参构造方法 int seed = 0; CodeEmitter e = ce.begin_method(Constants.ACC_PUBLIC, TypeUtils.parseConstructor(parameterTypes), null); e.load_this(); e.super_invoke_constructor(); e.load_this(); for (int i = 0; i < parameterTypes.length; i++) { seed += parameterTypes[i].hashCode(); //为每一个入参生成一个相同类型的类字段 ce.declare_field(Constants.ACC_PRIVATE | Constants.ACC_FINAL, getFieldName(i), parameterTypes[i], null); e.dup(); e.load_arg(i); e.putfield(getFieldName(i)); } e.return_value(); e.end_method(); //生成hashCode e = ce.begin_method(Constants.ACC_PUBLIC, HASH_CODE, null); int hc = (constant != 0) ? constant : PRIMES[(int)(Math.abs(seed) % PRIMES.length)]; int hm = (multiplier != 0) ? multiplier : PRIMES[(int)(Math.abs(seed * 13) % PRIMES.length)]; e.push(hc); for (int i = 0; i < parameterTypes.length; i++) { e.load_this(); e.getfield(getFieldName(i)); EmitUtils.hash_code(e, parameterTypes[i], hm, customizer); } e.return_value(); e.end_method(); //生成equals函数，在equals函数中对每个入参都进行判断 e = ce.begin_method(Constants.ACC_PUBLIC, EQUALS, null); Label fail = e.make_label(); e.load_arg(0); e.instance_of_this(); e.if_jump(e.EQ, fail); for (int i = 0; i < parameterTypes.length; i++) { e.load_this(); e.getfield(getFieldName(i)); e.load_arg(0); e.checkcast_this(); e.getfield(getFieldName(i)); EmitUtils.not_equals(e, parameterTypes[i], fail, customizer); } e.push(1); e.return_value(); e.mark(fail); e.push(0); e.return_value(); e.end_method(); //生成toString方法 e = ce.begin_method(Constants.ACC_PUBLIC, TO_STRING, null); e.new_instance(Constants.TYPE_STRING_BUFFER); e.dup(); e.invoke_constructor(Constants.TYPE_STRING_BUFFER); for (int i = 0; i < parameterTypes.length; i++) { if (i > 0) { e.push(", "); e.invoke_virtual(Constants.TYPE_STRING_BUFFER, APPEND_STRING); } e.load_this(); e.getfield(getFieldName(i)); EmitUtils.append_string(e, parameterTypes[i], EmitUtils.DEFAULT_DELIMITERS, customizer); } e.invoke_virtual(Constants.TYPE_STRING_BUFFER, TO_STRING); e.return_value(); e.end_method(); ce.end_class(); }
