本文提到的函數(function)和方法(method)為同一個意思

例子1，方法名的定義public?class?HalfRandom

{

public?static?double?f()

{

return?Math.random()/2;

}

}

編譯javac?HalfRandom.java

反編譯javap?-c?-verbose?HalfRandom.class...

major?version:?52

...

#2?=?Methodref??????????#16.#17????????//?java/lang/Math.random:()D

#3?=?Double?????????????2.0d

...

#12?=?Utf8???????????????()D

...

#16?=?Class??????????????#20????????????//?java/lang/Math

#17?=?NameAndType????????#21:#12????????//?random:()D

#18?=?Utf8???????????????HalfRandom

#19?=?Utf8???????????????java/lang/Object

#20?=?Utf8???????????????java/lang/Math

#21?=?Utf8???????????????random

...

public?static?double?f();

descriptor:?()D

flags:?ACC_PUBLIC,?ACC_STATIC

Code:

stack=4,?locals=0,?args_size=0

0:?invokestatic??#2??????????????????//?Method?java/lang/Math.random:()D

3:?ldc2_w????????#3??????????????????//?double?2.0d

6:?ddiv

7:?dreturn

LineNumberTable:

line?5:?0

invokestatic? #2? 調用常量#2定義的函數

函數名定義在常量池的Methodref中，它定義類,方法名,方法返回類型

常量#2中，可以看到它是由#16.#17拼接，#16定義了類名 java/lang/Math，#17是方法名和返回類型名random:()D

常量#17中，它是由#21:#12拼接，#21定義了方法名random，#12定義了返回類型名()D

()D的解釋 ,括號內沒有東西表示括號內無參數，D表示返回類型為double，如果是V則為void

這種方式

1)JVM可以檢查數據類型的正確性：

2)java反編譯器可以從被編譯的類文件中修改數據類型。

再看"hello，world！"的例子public?class?HelloWorld

{

public?static?void?main(String[]?args)

{

System.out.println("Hello,?World");

}

}

反編譯...

major?version:?52

...

#2?=?Fieldref???????????#16.#17????????//?java/lang/System.out:Ljava/io/PrintStream;

#3?=?String?????????????#18????????????//?Hello,?World

#4?=?Methodref??????????#19.#20????????//?java/io/PrintStream.println:(Ljava/lang/String;)V

...

#16?=?Class??????????????#23????????????//?java/lang/System

#17?=?NameAndType????????#24:#25????????//?out:Ljava/io/PrintStream;

#18?=?Utf8???????????????Hello,?World

#19?=?Class??????????????#26????????????//?java/io/PrintStream

#20?=?NameAndType????????#27:#28????????//?println:(Ljava/lang/String;)V

...

#23?=?Utf8???????????????java/lang/System

#24?=?Utf8???????????????out

#25?=?Utf8???????????????Ljava/io/PrintStream;

#26?=?Utf8???????????????java/io/PrintStream

#27?=?Utf8???????????????println

#28?=?Utf8???????????????(Ljava/lang/String;)V

...

public?static?void?main(java.lang.String[]);

descriptor:?([Ljava/lang/String;)V

flags:?ACC_PUBLIC,?ACC_STATIC

Code:

stack=2,?locals=1,?args_size=1

0:?getstatic?????#2??????????????????//?Field?java/lang/System.out:Ljava/io/PrintStream;

3:?ldc???????????#3??????????????????//?String?Hello,?World

5:?invokevirtual?#4??????????????????//?Method?java/io/PrintStream.println:(Ljava/lang/String;)V

8:?return

LineNumberTable:

line?5:?0

line?6:?8

getstatic ?#2 獲取System.out的引用入棧

ldc ?#3 將字符串Hello, World入棧

invokevirtual #4 調用println()方法，這里需要傳兩個參數，參數從棧中獲取，1先將Hello, World出棧傳入(Ljava/lang/String;)，2將System.out的引用出棧傳入java/io/PrintStream的引用

再看beep的函數調用(輸出計算機報警的蜂鳴聲)public?class?bee

{

public?static?void?main(String[]?args)

{

java.awt.Toolkit.getDefaultToolkit().beep();

}

}

反編譯...

major?version:?52

...

public?static?void?main(java.lang.String[]);

descriptor:?([Ljava/lang/String;)V

flags:?ACC_PUBLIC,?ACC_STATIC

Code:

stack=1,?locals=1,?args_size=1

0:?invokestatic??#2??????????????????//?Method?java/awt/Toolkit.getDefaultToolkit:()Ljava/awt/Toolkit;

3:?invokevirtual?#3??????????????????//?Method?java/awt/Toolkit.beep:()V

6:?return

LineNumberTable:

line?5:?0

line?6:?6

invokestatic ?#2 調用java.awt.Toolkit.getDefaultToolkit()方法并將返回結果的引用壓入棧頂

invokevirtual #3 調用beep()函數，其中需要傳引用，把objectref從棧頂彈出傳入java/awt/Toolkit

總結

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