从字节码角度分析泛型类，泛型方法，泛型接口的实现机制（类型擦除）
2017-07-23 21:35
567 查看
在java中泛型本质上是个语法糖，跟C++不一样，java根本不会创建泛型类，一切都只不过是编译器通过类型擦除机制实现的障眼法而已，比如
源码：
public
classChildTest<T> extends test {
Ta = (T)
"hello world";
public
void fun(T t){
System.out.println("ChildTest");
}
public ChildTest(){
}
/**
*
*@author : zhengrf1
*@date创建时间：2017年7月20日上午10:47:38
*/
public
static void main(String[]
args) {
// TODO Auto-generated method stub
ChildTest<String>mytest=
newChildTest<String>();
mytest.fun();
System.out.println(mytest.a.getClass());
}
}
字节码：
Last modified 2017-7-23; size 1074 bytes
MD5 checksum 25a0b1ff964cae6d640af43d89f72d7b
Compiled from "ChildTest.java"
public class ChildTest<T extends java.lang.Object> extendstest
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constantpool:
#1 = Class #2 // ChildTest
#2 = Utf8 ChildTest
#3 = Class #4 // test
#4 = Utf8 test
#5 = Utf8 a
#6 = Utf8 Ljava/lang/Object;
#7 = Utf8 Signature
#8 = Utf8 TT;
#9 = Utf8 fun
#10 = Utf8 (Ljava/lang/Object;)V
#11 = Utf8 (TT;)V
#12 = Utf8 Code
#13 = Fieldref #14.#16 // java/lang/System.out:Ljava/io/Print
Stream;
#14 = Class #15 // java/lang/System
#15 = Utf8 java/lang/System
#16 = NameAndType #17:#18 // out:Ljava/io/PrintStream;
#17 = Utf8 out
#18 = Utf8 Ljava/io/PrintStream;
#19 = String #2 // ChildTest
#20 = Methodref #21.#23 // java/io/PrintStream.println:(Ljava/
lang/String;)V
#21 = Class #22 // java/io/PrintStream
#22 = Utf8 java/io/PrintStream
#23 = NameAndType #24:#25 // println:(Ljava/lang/String;)V
#24 = Utf8 println
#25 = Utf8 (Ljava/lang/String;)V
#26 = Utf8 LineNumberTable
#27 = Utf8 LocalVariableTable
#28 = Utf8 this
#29 = Utf8 LChildTest;
#30 = Utf8 t
#31 = Utf8 LocalVariableTypeTable
#32 = Utf8 LChildTest<TT;>;
#33 = Utf8 <init>
#34 = Utf8 ()V
#35 = Methodref #3.#36 // test."<init>":()V
#36 = NameAndType #33:#34 // "<init>":()V
#37 = String #38 // hello world
#38 = Utf8 hello world
#39 = Fieldref #1.#40 // ChildTest.a:Ljava/lang/Object;
#40 = NameAndType #5:#6 // a:Ljava/lang/Object;
#41 = Utf8 main
#42 = Utf8 ([Ljava/lang/String;)V
#43 = Methodref #1.#36 //ChildTest."<init>":()V
#44 = Methodref #1.#45 // ChildTest.fun:()V
#45 = NameAndType #9:#34 // fun:()V
#46 = Class #47 // java/lang/String
#47 = Utf8 java/lang/String
#48 = Methodref #49.#51 // java/lang/Object.getClass:()Ljava/l
ang/Class;
#49 = Class #50 // java/lang/Object
#50 = Utf8 java/lang/Object
#51 = NameAndType #52:#53 // getClass:()Ljava/lang/Class;
#52 = Utf8 getClass
#53 = Utf8 ()Ljava/lang/Class;
#54 = Methodref #21.#55 // java/io/PrintStream.println:(Ljava/
lang/Object;)V
#55 = NameAndType #24:#10 // println:(Ljava/lang/Object;)V
#56 = Utf8 args
#57 = Utf8 [Ljava/lang/String;
#58 = Utf8 mytest
#59 = Utf8 LChildTest<Ljava/lang/String;>;
#60 = Utf8 SourceFile
#61 = Utf8 ChildTest.java
#62 = Utf8 <T:Ljava/lang/Object;>Ltest;
{
T a;
descriptor:Ljava/lang/Object;
flags:
Signature: #8 // TT;
public void fun(T);
descriptor:(Ljava/lang/Object;)V
flags: ACC_PUBLIC
Signature: #11 // (TT;)V
Code:
stack=2, locals=2, args_size=2
0: getstatic #13 // Fieldjava/lang/System.out:Ljav
a/io/PrintStream;
3: ldc #19 // String ChildTest
5: invokevirtual #20 // Methodjava/io/PrintStream.prin
tln:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 19: 0
line 20: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this LChildTest;
0 9 1 t Ljava/lang/Object;
LocalVariableTypeTable:
Start Length Slot Name Signature
0 9 0 this LChildTest<TT;>;
0 9 1 t TT;
public ChildTest();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #35 // Methodtest."<init>":()V
4: aload_0
5: ldc #37 // String hello world
7: putfield #39 // Fielda:Ljava/lang/Object;
10: return
LineNumberTable:
line 21: 0
line 16: 4
line 23: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this LChildTest;
LocalVariableTypeTable:
Start Length Slot Name Signature
0 11 0 this LChildTest<TT;>;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: new #1 // class ChildTest
3: dup
4: invokespecial #43 // Method"<init>":()V
7: astore_1
8: aload_1
9: invokevirtual #44 // Method fun:()V
12: getstatic #13 // Fieldjava/lang/System.out:Ljav
a/io/PrintStream;
15: aload_1
16: getfield #39 // Field a:Ljava/lang/Object;
19: checkcast #46 // class java/lang/String
22: invokevirtual#48 // Methodjava/lang/Object.getClas
s:()Ljava/lang/Class;
25: invokevirtual #54 // Methodjava/io/PrintStream.prin
tln:(Ljava/lang/Object;)V
28: return
LineNumberTable:
line 32: 0
line 33: 8
line 34: 12
line 35: 28
LocalVariableTable:
Start Length Slot Name Signature
0 29 0 args [Ljava/lang/String;
8 21 1 mytest LChildTest;
LocalVariableTypeTable:
Start Length Slot Name Signature
8 21 1 mytest LChildTest<Ljava/lang/String;>;
}
--注意红色部分，可见在编译成字节码时编译器把所有T类型替换成上帝类Object，那么使用时是怎么根据真正传入的类型String调用String类的相关方法的呢？看绿色部分，当我们真正用到T a这个成员时，编译器在编译时会在代码前面加上类型转换检查指令checkcast，把a转化成String类型。然后再调用String类型的相关方法，这种编译器私自添加向下类型转换机制便称为泛型的类型擦除机制。以此完成语法糖的功能。
源码：
public
classChildTest<T> extends test {
Ta = (T)
"hello world";
public
void fun(T t){
System.out.println("ChildTest");
}
public ChildTest(){
}
/**
*
*@author : zhengrf1
*@date创建时间：2017年7月20日上午10:47:38
*/
public
static void main(String[]
args) {
// TODO Auto-generated method stub
ChildTest<String>mytest=
newChildTest<String>();
mytest.fun();
System.out.println(mytest.a.getClass());
}
}
字节码：
Last modified 2017-7-23; size 1074 bytes
MD5 checksum 25a0b1ff964cae6d640af43d89f72d7b
Compiled from "ChildTest.java"
public class ChildTest<T extends java.lang.Object> extendstest
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constantpool:
#1 = Class #2 // ChildTest
#2 = Utf8 ChildTest
#3 = Class #4 // test
#4 = Utf8 test
#5 = Utf8 a
#6 = Utf8 Ljava/lang/Object;
#7 = Utf8 Signature
#8 = Utf8 TT;
#9 = Utf8 fun
#10 = Utf8 (Ljava/lang/Object;)V
#11 = Utf8 (TT;)V
#12 = Utf8 Code
#13 = Fieldref #14.#16 // java/lang/System.out:Ljava/io/Print
Stream;
#14 = Class #15 // java/lang/System
#15 = Utf8 java/lang/System
#16 = NameAndType #17:#18 // out:Ljava/io/PrintStream;
#17 = Utf8 out
#18 = Utf8 Ljava/io/PrintStream;
#19 = String #2 // ChildTest
#20 = Methodref #21.#23 // java/io/PrintStream.println:(Ljava/
lang/String;)V
#21 = Class #22 // java/io/PrintStream
#22 = Utf8 java/io/PrintStream
#23 = NameAndType #24:#25 // println:(Ljava/lang/String;)V
#24 = Utf8 println
#25 = Utf8 (Ljava/lang/String;)V
#26 = Utf8 LineNumberTable
#27 = Utf8 LocalVariableTable
#28 = Utf8 this
#29 = Utf8 LChildTest;
#30 = Utf8 t
#31 = Utf8 LocalVariableTypeTable
#32 = Utf8 LChildTest<TT;>;
#33 = Utf8 <init>
#34 = Utf8 ()V
#35 = Methodref #3.#36 // test."<init>":()V
#36 = NameAndType #33:#34 // "<init>":()V
#37 = String #38 // hello world
#38 = Utf8 hello world
#39 = Fieldref #1.#40 // ChildTest.a:Ljava/lang/Object;
#40 = NameAndType #5:#6 // a:Ljava/lang/Object;
#41 = Utf8 main
#42 = Utf8 ([Ljava/lang/String;)V
#43 = Methodref #1.#36 //ChildTest."<init>":()V
#44 = Methodref #1.#45 // ChildTest.fun:()V
#45 = NameAndType #9:#34 // fun:()V
#46 = Class #47 // java/lang/String
#47 = Utf8 java/lang/String
#48 = Methodref #49.#51 // java/lang/Object.getClass:()Ljava/l
ang/Class;
#49 = Class #50 // java/lang/Object
#50 = Utf8 java/lang/Object
#51 = NameAndType #52:#53 // getClass:()Ljava/lang/Class;
#52 = Utf8 getClass
#53 = Utf8 ()Ljava/lang/Class;
#54 = Methodref #21.#55 // java/io/PrintStream.println:(Ljava/
lang/Object;)V
#55 = NameAndType #24:#10 // println:(Ljava/lang/Object;)V
#56 = Utf8 args
#57 = Utf8 [Ljava/lang/String;
#58 = Utf8 mytest
#59 = Utf8 LChildTest<Ljava/lang/String;>;
#60 = Utf8 SourceFile
#61 = Utf8 ChildTest.java
#62 = Utf8 <T:Ljava/lang/Object;>Ltest;
{
T a;
descriptor:Ljava/lang/Object;
flags:
Signature: #8 // TT;
public void fun(T);
descriptor:(Ljava/lang/Object;)V
flags: ACC_PUBLIC
Signature: #11 // (TT;)V
Code:
stack=2, locals=2, args_size=2
0: getstatic #13 // Fieldjava/lang/System.out:Ljav
a/io/PrintStream;
3: ldc #19 // String ChildTest
5: invokevirtual #20 // Methodjava/io/PrintStream.prin
tln:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 19: 0
line 20: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this LChildTest;
0 9 1 t Ljava/lang/Object;
LocalVariableTypeTable:
Start Length Slot Name Signature
0 9 0 this LChildTest<TT;>;
0 9 1 t TT;
public ChildTest();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #35 // Methodtest."<init>":()V
4: aload_0
5: ldc #37 // String hello world
7: putfield #39 // Fielda:Ljava/lang/Object;
10: return
LineNumberTable:
line 21: 0
line 16: 4
line 23: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this LChildTest;
LocalVariableTypeTable:
Start Length Slot Name Signature
0 11 0 this LChildTest<TT;>;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: new #1 // class ChildTest
3: dup
4: invokespecial #43 // Method"<init>":()V
7: astore_1
8: aload_1
9: invokevirtual #44 // Method fun:()V
12: getstatic #13 // Fieldjava/lang/System.out:Ljav
a/io/PrintStream;
15: aload_1
16: getfield #39 // Field a:Ljava/lang/Object;
19: checkcast #46 // class java/lang/String
22: invokevirtual#48 // Methodjava/lang/Object.getClas
s:()Ljava/lang/Class;
25: invokevirtual #54 // Methodjava/io/PrintStream.prin
tln:(Ljava/lang/Object;)V
28: return
LineNumberTable:
line 32: 0
line 33: 8
line 34: 12
line 35: 28
LocalVariableTable:
Start Length Slot Name Signature
0 29 0 args [Ljava/lang/String;
8 21 1 mytest LChildTest;
LocalVariableTypeTable:
Start Length Slot Name Signature
8 21 1 mytest LChildTest<Ljava/lang/String;>;
}
--注意红色部分，可见在编译成字节码时编译器把所有T类型替换成上帝类Object，那么使用时是怎么根据真正传入的类型String调用String类的相关方法的呢？看绿色部分，当我们真正用到T a这个成员时，编译器在编译时会在代码前面加上类型转换检查指令checkcast，把a转化成String类型。然后再调用String类型的相关方法，这种编译器私自添加向下类型转换机制便称为泛型的类型擦除机制。以此完成语法糖的功能。
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