在阅读spring源码时,看到桥接方法,因此这里将整理的文档贴出来
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/////////////////////////////////////////////桥接方法: Bridge method/////////////////////////////////////////////////
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什么是桥接方法? 根据JVM规范(https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.6)的描述是:
桥接方法是由编译器生成.
编译器什么时候会生成桥接方法呢?
目前只知道实现泛型接口的类编译器会生成桥接方法
例如:
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|public class StringTest implements Test<String> {
| //Test类是泛型类或接口:public class Test<T>{// 方法参数含有T}
| @Override
| public String get() {
| return null;
| }
|
| @Override
| public void add(String s) {
|
| }
|}
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测试代码:
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| Method[] methods = StringTest.class.getDeclaredMethods();
| for (Method m : methods){
| Class[] cld = m.getParameterTypes();
| StringBuilder out = new StringBuilder(m.getName()).append("(");
| for (Class cl : cld){
| out.append(cl.getSimpleName()).append(",");
| }
|
| int index = out.lastIndexOf(",");
| if(index != -1){
| out.replace(index,index+1,"");
| }
| out.append(")");
| System.out.println(out+"------isBridge:"+m.isBridge()+"------isSynthetic:"+m.isSynthetic());
| }
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输出结果:
|--------------------------------------------------------------------------------------------------------------------
| add(Object)------isBridge:true------isSynthetic:true --// 编译器生成
| add(String)------isBridge:false------isSynthetic:false
| get()------isBridge:false------isSynthetic:false
| get()------isBridge:true------isSynthetic:true --// 编译器生成
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查看字节码
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通过命令: javap -v StringTest.class
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public java.lang.String get();
descriptor: ()Ljava/lang/String;
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aconst_null
1: areturn
LineNumberTable:
line 9: 0
LocalVariableTable:
Start Length Slot Name Signature
0 2 0 this Lcom/optimus/utils/StringTest;
public void add(java.lang.String);
descriptor: (Ljava/lang/String;)V
flags: ACC_PUBLIC
Code:
stack=0, locals=2, args_size=2
0: return
LineNumberTable:
line 15: 0
LocalVariableTable:
Start Length Slot Name Signature
0 1 0 this Lcom/optimus/utils/StringTest;
0 1 1 s Ljava/lang/String;
public void add(java.lang.Object);
descriptor: (Ljava/lang/Object;)V
flags: ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC // ACC_BRIDGE:桥接方法的标志,此时通过Method.isBridge()返回true
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: aload_1
2: checkcast #2 // class java/lang/String
5: invokevirtual #3 // Method add:(Ljava/lang/String;)V --桥接方法中调用了实际的方法(出现在源代码中的方法)
8: return
LineNumberTable:
line 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this Lcom/optimus/utils/StringTest;
public java.lang.Object get();
descriptor: ()Ljava/lang/Object;
flags: ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokevirtual #4 // Method get:()Ljava/lang/String; --桥接方法中调用了 实际的方法(出现在源代码中的方法)
4: areturn
LineNumberTable:
line 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/optimus/utils/StringTest;
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由字节应该可以知道:
Test test = new StringTest();
test.add("123"); // 实际调用的是StringTest.add(String)方法
test.add(new Object()) // 调用的是桥接方法,因桥接方法中会调用实际方法,因此调用会抛出类型转换异常
对于泛型接口而言,参数的不确定性,是编译器生成桥接方法的原因,因为接口的对外的使用方式是统一的,不同的只是实现,
因此在实现类中要生成桥接方法,以适应通过接口的调用,
在运行时通过动态绑定获知具体类型,进行参数强转调用实现类的方法。若没有桥接接口,编译期检查应当都不会通过的。
这里所讨论的泛型不是<T extends TestClass> 这种形式的,因为这种形式是可以确定参数类型,编译后参数的类型就是 TestClass
至于Synthetic标识,标识代码是由编译器生成而非由源码(程序员所写)编译:
按此说法 动态代理,内部类,泛型接口实现类等均会有此标识,本人只测试了泛型接口实现类和内部类,结果是如此。
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///////////////////////////////////////////////////资源搜索//////////////////////////////////////////////////////////
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|Class.getResource(String name) vs Class.getResourceAsStream(String name)
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|两者的搜索路径相同,具体如下:
|name以'/'开始,则是从classpath下进行搜索
|name不以'/'开始,则是根据当前class对象包名所表示的路径下搜索:
|(eg: com.cgtz.optimus.test.Test.class,搜索路径为:com/cgtz/optimus/test/name)
|
|两者方法内部在进行搜索前会调用 resolveName()方法:
|-------------------------------------------------------------------------------------------------------------------
|private String resolveName(String name) {
| if (name == null) {
| return name;
| }
| if (!name.startsWith("/")) {
| Class<?> c = this;
| while (c.isArray()) {
| c = c.getComponentType();
| }
| String baseName = c.getName();
| int index = baseName.lastIndexOf('.');
| if (index != -1) {
| name = baseName.substring(0, index).replace('.', '/')
| +"/"+name;
| }
| } else {
| name = name.substring(1);
| }
| return name;
|}
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|ClassLoader.getResource(String name) vs ClassLoader.getResourceAsStream(String name)
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|
|两者搜索路径亦是相同,与Class中的不同,ClassLoader是在classpath下开始搜索,即name不能以'/'开头,否则返回的永远是Null
|
|其实上述Class的两个方法内部是先进行resolveName(), 然后调用ClassLoader.getResource或ClassLoader.getResourceAsStream
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