如何通过字节码看java中this的隐式传参
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前言
从字节码看java中 this 隐式传参具体体现（和python中的self如出一辙，但是比python中藏得更深），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。
概述
this关键字，是一个隐式参数，另外一个隐式参数是super。
this用于方法里面，用于方法外面无意义。
this关键字一般用于set方法和构造方法中。
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);
}
}反汇编命令：
javap -verbose Hello.class
反汇编结果：
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."":()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." ":()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 #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 // " ":()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." ":()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 10 0 this Lcom/xx/api/Hello; 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 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." ":()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 26 0 this Lcom/xx/api/Hello; 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." ":()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 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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