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从字节码看java中 this 隐式传参具体体现
source link: http://www.cnblogs.com/yougewe/p/9929249.html?amp%3Butm_medium=referral
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从字节码看java中 this 隐式传参具体体现，也发现了 static 与 非 static 方法的区别所在！
static与非static方法都是存储java的方法区。在static 方法中，没有this引用，因此无法使用当前类中所定义的变量，而非static方法则会默认传入this。如果你在其他地方看到过这个理论，那么我们今天就从另一个角度来真实看一下这个答案吧！
来个例子，并将其反编译为可视代码:
public class Hello {
private final int ii;
public Hello(int a) {
ii = a;
}
public static void main(String[] args) throws Exception {
sayHelloStatic("ok");
}
public void sayHello(String word) {
System.out.println("hello, " + word);
}
public static void sayHelloStatic(String word) {
System.out.println("static hello, " + word);
}
}
反汇编：
javap -verbose Hello.class
Classfile /D:/xx/target/classes/com/xx/api/Hello.class
Last modified 2018-11-8; size 1069 bytes
MD5 checksum 9d39cd9d4e95588a73c059a4e69f01e8
Compiled from "Hello.java"
public class com.xx.api.Hello
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #14.#38 // java/lang/Object."<init>":()V
#2 = Fieldref #13.#39 // com/xx/api/Hello.ii:I
#3 = String #40 // ok
#4 = Methodref #13.#41 // com/xx/api/Hello.sayHelloStatic:(Ljava/lang/String;)V
#5 = Fieldref #42.#43 // java/lang/System.out:Ljava/io/PrintStream;
#6 = Class #44 // java/lang/StringBuilder
#7 = Methodref #6.#38 // java/lang/StringBuilder."<init>":()V
#8 = String #45 // hello,
#9 = Methodref #6.#46 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#10 = Methodref #6.#47 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#11 = Methodref #48.#49 // java/io/PrintStream.println:(Ljava/lang/String;)V
#12 = String #50 // static hello,
#13 = Class #51 // com/xx/api/Hello
#14 = Class #52 // java/lang/Object
#15 = Utf8 ii
#16 = Utf8 I
#17 = Utf8 <init>
#18 = Utf8 (I)V
#19 = Utf8 Code
#20 = Utf8 LineNumberTable
#21 = Utf8 LocalVariableTable
#22 = Utf8 this
#23 = Utf8 Lcom/xx/api/Hello;
#24 = Utf8 a
#25 = Utf8 main
#26 = Utf8 ([Ljava/lang/String;)V
#27 = Utf8 args
#28 = Utf8 [Ljava/lang/String;
#29 = Utf8 Exceptions
#30 = Class #53 // java/lang/Exception
#31 = Utf8 sayHello
#32 = Utf8 (Ljava/lang/String;)V
#33 = Utf8 word
#34 = Utf8 Ljava/lang/String;
#35 = Utf8 sayHelloStatic
#36 = Utf8 SourceFile
#37 = Utf8 Hello.java
#38 = NameAndType #17:#54 // "<init>":()V
#39 = NameAndType #15:#16 // ii:I
#40 = Utf8 ok
#41 = NameAndType #35:#32 // sayHelloStatic:(Ljava/lang/String;)V
#42 = Class #55 // java/lang/System
#43 = NameAndType #56:#57 // out:Ljava/io/PrintStream;
#44 = Utf8 java/lang/StringBuilder
#45 = Utf8 hello,
#46 = NameAndType #58:#59 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#47 = NameAndType #60:#61 // toString:()Ljava/lang/String;
#48 = Class #62 // java/io/PrintStream
#49 = NameAndType #63:#32 // println:(Ljava/lang/String;)V
#50 = Utf8 static hello,
#51 = Utf8 com/xx/api/Hello
#52 = Utf8 java/lang/Object
#53 = Utf8 java/lang/Exception
#54 = Utf8 ()V
#55 = Utf8 java/lang/System
#56 = Utf8 out
#57 = Utf8 Ljava/io/PrintStream;
#58 = Utf8 append
#59 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#60 = Utf8 toString
#61 = Utf8 ()Ljava/lang/String;
#62 = Utf8 java/io/PrintStream
#63 = Utf8 println
{
public com.xx.api.Hello(int);
descriptor: (I)V
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: iload_1
6: putfield #2 // Field ii:I
9: return
LineNumberTable:
line 14: 0
line 15: 4
line 16: 9
LocalVariableTable:
Start Length Slot Name Signature
0 10 0 this Lcom/xx/api/Hello;
0 10 1 a I
public static void main(java.lang.String[]) throws java.lang.Exception;
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=1, args_size=1
0: ldc #3 // String ok
2: invokestatic #4 // Method sayHelloStatic:(Ljava/lang/String;)V
5: return
LineNumberTable:
line 42: 0
line 45: 5
LocalVariableTable:
Start Length Slot Name Signature
0 6 0 args [Ljava/lang/String;
Exceptions:
throws java.lang.Exception
public void sayHello(java.lang.String);
descriptor: (Ljava/lang/String;)V
flags: ACC_PUBLIC
Code:
stack=3, locals=2, args_size=2
0: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;
3: new #6 // class java/lang/StringBuilder
6: dup
7: invokespecial #7 // Method java/lang/StringBuilder."<init>":()V
10: ldc #8 // String hello,
12: invokevirtual #9 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
15: aload_1
16: invokevirtual #9 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
22: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
25: return
LineNumberTable:
line 48: 0
line 49: 25
LocalVariableTable:
Start Length Slot Name Signature
0 26 0 this Lcom/xx/api/Hello;
0 26 1 word Ljava/lang/String;
public static void sayHelloStatic(java.lang.String);
descriptor: (Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=1, args_size=1
0: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;
3: new #6 // class java/lang/StringBuilder
6: dup
7: invokespecial #7 // Method java/lang/StringBuilder."<init>":()V
10: ldc #12 // String static hello,
12: invokevirtual #9 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
15: aload_0
16: invokevirtual #9 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: invokevirtual #10 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
22: invokevirtual #11 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
25: return
LineNumberTable:
line 51: 0
line 52: 25
LocalVariableTable:
Start Length Slot Name Signature
0 26 0 word Ljava/lang/String;
}
SourceFile: "Hello.java"
我们从字节码文件中可以看出来：
sayHello(String word) 和 sayHelloStatic(String word) 都只有一个参数，但是在字节码中：
sayHello(String word) 中引用 word 时使用了 15: aload_1, 可以看出其加载的变量是在 slot1中，而 slot0中即保存了 this 。
sayHelloStatic(String word) 中引用 word 时使用了 15: aload_0, 可以看出静态方法中，直接将变量存在了 slot0中，因此无法使用 this 中的变量了。
当要操作当前类的变量或方法时，需要先 aload_0, 然后再做相关操作！
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