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這篇文章主要講解了“Java的javap工具怎么使用”，文中的講解內(nèi)容簡單清晰，易于學(xué)習(xí)與理解，下面請(qǐng)大家跟著小編的思路慢慢深入，一起來研究和學(xué)習(xí)“Java的javap工具怎么使用”吧！
一：簡介
javap是JDK自帶的反匯編器，可以查看java編譯器為我們生成的字節(jié)碼。通過它，我們可以對(duì)照源代碼和字節(jié)碼，從而了解很多編譯器內(nèi)部的工作。
javap命令的常用參數(shù)有：
-l 打印行和本地變量表
-public 只顯示公共類和成員變量
-protected 只顯示公共的、受保護(hù)的類和變量
-private 顯示所有的類和成員變量
-p 顯示所有的類和成員變量
-Joption 給jvm指定參數(shù),例如:
javap -J-version
javap -J-Djava.security.manager -J-Djava.security.policy=MyPolicy
-s 打印內(nèi)部類型簽名(用于泛型)
-sysinfo 打印被處理類的系統(tǒng)信息(路徑,大小,日志,MD5哈希)
-constants 顯示static final常量
-c 輸出分解后的代碼。例如，類中每一個(gè)方法內(nèi)，包含java字節(jié)碼的指令
-verbose 輸出棧大小，變量（locals）和方法參數(shù)的個(gè)數(shù)
-classpath path 指定查詢類的路徑.如果設(shè)置該值,則會(huì)覆蓋默認(rèn)值和CLASSPATH環(huán)境變量。
-bootclasspath path 指定啟動(dòng)類的路徑,默認(rèn)值是jre/lib/rt.jar 和 其他少數(shù)的jar.
-extdir dirs 覆蓋 java.ext.dirs路徑
二：實(shí)例
1.Java源代碼
public class Demo {
private String name = "張三";
public static void main(String args[]) {
System.out.println("Hello World.");
}
private void sayHello() {
System.out.println("Hello " + name);
}
}2.javac編譯后的字節(jié)碼
CA FE BA BE 00 00 00 34 00 36 0A 00 0D 00 1E 08 00 1F 09 00 0C 00 20 09 00 21 00 22 08 00 23 0A 00 24 00 25 07 00 26 0A 00 07 00 1E 08 00 27 0A 00 07 00 28 0A 00 07 00 29 07 00 2A 07 00 2B 01 00 04 6E 61 6D 65 01 00 12 4C 6A 61 76 61 2F 6C 61 6E 67 2F 53 74 72 69 6E 67 3B 01 00 06 3C 69 6E 69 74 3E 01 00 03 28 29 56 01 00 04 43 6F 64 65 01 00 0F 4C 69 6E 65 4E 75 6D 62 65 72 54 61 62 6C 65 01 00 12 4C 6F 63 61 6C 56 61 72 69 61 62 6C 65 54 61 62 6C 65 01 00 04 74 68 69 73 01 00 28 4C 63 6F 6D 2F 62 72 69 6C 6C 69 61 6E 74 73 74 61 72 2F 63 69 6D 63 2F 69 6D 2F 64 65 73 6B 74 6F 70 2F 44 65 6D 6F 3B 01 00 04 6D 61 69 6E 01 00 16 28 5B 4C 6A 61 76 61 2F 6C 61 6E 67 2F 53 74 72 69 6E 67 3B 29 56 01 00 04 61 72 67 73 01 00 13 5B 4C 6A 61 76 61 2F 6C 61 6E 67 2F 53 74 72 69 6E 67 3B 01 00 08 73 61 79 48 65 6C 6C 6F 01 00 0A 53 6F 75 72 63 65 46 69 6C 65 01 00 09 44 65 6D 6F 2E 6A 61 76 61 0C 00 10 00 11 01 00 06 E5 BC A0 E4 B8 89 0C 00 0E 00 0F 07 00 2C 0C 00 2D 00 2E 01 00 0C 48 65 6C 6C 6F 20 57 6F 72 6C 64 2E 07 00 2F 0C 00 30 00 31 01 00 17 6A 61 76 61 2F 6C 61 6E 67 2F 53 74 72 69 6E 67 42 75 69 6C 64 65 72 01 00 06 48 65 6C 6C 6F 20 0C 00 32 00 33 0C 00 34 00 35 01 00 26 63 6F 6D 2F 62 72 69 6C 6C 69 61 6E 74 73 74 61 72 2F 63 69 6D 63 2F 69 6D 2F 64 65 73 6B 74 6F 70 2F 44 65 6D 6F 01 00 10 6A 61 76 61 2F 6C 61 6E 67 2F 4F 62 6A 65 63 74 01 00 10 6A 61 76 61 2F 6C 61 6E 67 2F 53 79 73 74 65 6D 01 00 03 6F 75 74 01 00 15 4C 6A 61 76 61 2F 69 6F 2F 50 72 69 6E 74 53 74 72 65 61 6D 3B 01 00 13 6A 61 76 61 2F 69 6F 2F 50 72 69 6E 74 53 74 72 65 61 6D 01 00 07 70 72 69 6E 74 6C 6E 01 00 15 28 4C 6A 61 76 61 2F 6C 61 6E 67 2F 53 74 72 69 6E 67 3B 29 56 01 00 06 61 70 70 65 6E 64 01 00 2D 28 4C 6A 61 76 61 2F 6C 61 6E 67 2F 53 74 72 69 6E 67 3B 29 4C 6A 61 76 61 2F 6C 61 6E 67 2F 53 74 72 69 6E 67 42 75 69 6C 64 65 72 3B 01 00 08 74 6F 53 74 72 69 6E 67 01 00 14 28 29 4C 6A 61 76 61 2F 6C 61 6E 67 2F 53 74 72 69 6E 67 3B 00 21 00 0C 00 0D 00 00 00 01 00 02 00 0E 00 0F 00 00 00 03 00 01 00 10 00 11 00 01 00 12 00 00 00 39 00 02 00 01 00 00 00 0B 2A B7 00 01 2A 12 02 B5 00 03 B1 00 00 00 02 00 13 00 00 00 0A 00 02 00 00 00 13 00 04 00 14 00 14 00 00 00 0C 00 01 00 00 00 0B 00 15 00 16 00 00 00 09 00 17 00 18 00 01 00 12 00 00 00 37 00 02 00 01 00 00 00 09 B2 00 04 12 05 B6 00 06 B1 00 00 00 02 00 13 00 00 00 0A 00 02 00 00 00 17 00 08 00 18 00 14 00 00 00 0C 00 01 00 00 00 09 00 19 00 1A 00 00 00 02 00 1B 00 11 00 01 00 12 00 00 00 4B 00 03 00 01 00 00 00 1D B2 00 04 BB 00 07 59 B7 00 08 12 09 B6 00 0A 2A B4 00 03 B6 00 0A B6 00 0B B6 00 06 B1 00 00 00 02 00 13 00 00 00 0A 00 02 00 00 00 1B 00 1C 00 1C 00 14 00 00 00 0C 00 01 00 00 00 1D 00 15 00 16 00 00 00 01 00 1C 00 00 00 02 00 1D
3.javap返匯編后的內(nèi)容
javap -p -v Demo
默認(rèn)情況下javap會(huì)打印所有非私有的字段和方法，當(dāng)加了-p選項(xiàng)后，它還將打印私有的字段和方法；-v 它盡可能地打印所有信息。如果只需要查詢方法對(duì)應(yīng)的字節(jié)碼，那么可以用-c代替-v.
=================基本信息(涵蓋了原class文件的相關(guān)信息)=======================
Classfile /D:/workspace/donkey/cimc-im-desktop/target/classes/com/brilliantstar/cimc/im/desktop/Demo.class
Last modified 2018-9-6; size 909 bytes
MD5 checksum 0f9cd841c4a2ab1f6a00163e55ca6e0f
Compiled from "Demo.java"
public class com.brilliantstar.cimc.im.desktop.Demo
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
=================常量池(用來存放各種常量及符號(hào)引用)=======================
Constant pool:
#1 = Methodref #13.#30 // java/lang/Object."<init>":()V
#2 = String #31 // 張三
#3 = Fieldref #12.#32 // com/brilliantstar/cimc/im/desktop/Demo.name:Ljava/lang/String;
#4 = Fieldref #33.#34 // java/lang/System.out:Ljava/io/PrintStream;
#5 = String #35 // Hello World.
#6 = Methodref #36.#37 // java/io/PrintStream.println:(Ljava/lang/String;)V
#7 = Class #38 // java/lang/StringBuilder
#8 = Methodref #7.#30 // java/lang/StringBuilder."<init>":()V
#9 = String #39 // Hello
#10 = Methodref #7.#40 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#11 = Methodref #7.#41 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#12 = Class #42 // com/brilliantstar/cimc/im/desktop/Demo
#13 = Class #43 // java/lang/Object
#14 = Utf8 name
#15 = Utf8 Ljava/lang/String;
#16 = Utf8 <init>
#17 = Utf8 ()V
#18 = Utf8 Code
#19 = Utf8 LineNumberTable
#20 = Utf8 LocalVariableTable
#21 = Utf8 this
#22 = Utf8 Lcom/brilliantstar/cimc/im/desktop/Demo;
#23 = Utf8 main
#24 = Utf8 ([Ljava/lang/String;)V
#25 = Utf8 args
#26 = Utf8 [Ljava/lang/String;
#27 = Utf8 sayHello
#28 = Utf8 SourceFile
#29 = Utf8 Demo.java
#30 = NameAndType #16:#17 // "<init>":()V
#31 = Utf8 張三
#32 = NameAndType #14:#15 // name:Ljava/lang/String;
#33 = Class #44 // java/lang/System
#34 = NameAndType #45:#46 // out:Ljava/io/PrintStream;
#35 = Utf8 Hello World.
#36 = Class #47 // java/io/PrintStream
#37 = NameAndType #48:#49 // println:(Ljava/lang/String;)V
#38 = Utf8 java/lang/StringBuilder
#39 = Utf8 Hello
#40 = NameAndType #50:#51 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#41 = NameAndType #52:#53 // toString:()Ljava/lang/String;
#42 = Utf8 com/brilliantstar/cimc/im/desktop/Demo
#43 = Utf8 java/lang/Object
#44 = Utf8 java/lang/System
#45 = Utf8 out
#46 = Utf8 Ljava/io/PrintStream;
#47 = Utf8 java/io/PrintStream
#48 = Utf8 println
#49 = Utf8 (Ljava/lang/String;)V
#50 = Utf8 append
#51 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#52 = Utf8 toString
#53 = Utf8 ()Ljava/lang/String;
{
===============字段區(qū)域(用來列舉該類中的各個(gè)字段)===================
private java.lang.String name;
descriptor: Ljava/lang/String;
flags: ACC_PRIVATE
public com.brilliantstar.cimc.im.desktop.Demo();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String 張三
7: putfield #3 // Field name:Ljava/lang/String;
10: return
LineNumberTable:
line 19: 0
line 20: 4
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this Lcom/brilliantstar/cimc/im/desktop/Demo;
===============方法區(qū)域(用來列舉該類中的各個(gè)方法)=========================
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 #4 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #5 // String Hello World.
5: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 23: 0
line 24: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args [Ljava/lang/String;
private void sayHello();
descriptor: ()V
flags: ACC_PRIVATE
Code:
stack=3, locals=1, args_size=1
0: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
3: new #7 // class java/lang/StringBuilder
6: dup
7: invokespecial #8 // Method java/lang/StringBuilder."<init>":()V
10: ldc #9 // String Hello
12: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
15: aload_0
16: getfield #3 // Field name:Ljava/lang/String;
19: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: invokevirtual #11 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
25: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
28: return
LineNumberTable:
line 27: 0
line 28: 28
LocalVariableTable:
Start Length Slot Name Signature
0 29 0 this Lcom/brilliantstar/cimc/im/desktop/Demo;
}
SourceFile: "Demo.java"以其中的sayHello()方法為例，除了方法描述以及訪問權(quán)限之外，方法中還包括最為重要的代碼區(qū)域。
1).代碼區(qū)域一開始會(huì)聲明該方法中的操作數(shù)棧(stack=3) 和局部變量數(shù)目(locals=1)的最大值，以及該方法接收參數(shù)的個(gè)數(shù)(args_size=1)這里局部變量指的是字節(jié)碼中的局部變量，而非Java程序中的局部變量。
2).接下來是該方法的字節(jié)碼，每條字節(jié)碼均標(biāo)注了對(duì)應(yīng)的偏移量，這是用來定位字節(jié)碼的。
3).接下來的行數(shù)表(LineNumberTable)則是Java源程序到字節(jié)碼偏移量的映射。行數(shù)表和局部變量均屬于調(diào)試信息，Java虛擬機(jī)并不要求class文件必備這些信息。
感謝各位的閱讀，以上就是“Java的javap工具怎么使用”的內(nèi)容了，經(jīng)過本文的學(xué)習(xí)后，相信大家對(duì)Java的javap工具怎么使用這一問題有了更深刻的體會(huì)，具體使用情況還需要大家實(shí)踐驗(yàn)證。這里是億速云，小編將為大家推送更多相關(guān)知識(shí)點(diǎn)的文章，歡迎關(guān)注！
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