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:
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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:
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javap -verbose hello. class |
反汇编结果:
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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 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."<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 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."<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 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!
总结:
以上就是这篇文章的全部内容了,希望本文的内容对大家的学习或者工作具有一定的参考学习价值,如果有疑问大家可以留言交流,谢谢大家对服务器之家的支持。
原文链接:http://www.cnblogs.com/yougewe/p/9929249.html