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
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
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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，谢谢大家对快网idc的支持。
原文链接：http://www.cnblogs.com/yougewe/p/9929249.html
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