软件工程师-Java字节码开发深入解析教程解释.docx

本文将对Java字节码进行解析，以及一些效率问题。AD： 一：Java字节代码的组织形式类文件OxCAFEBABE，小版本号，大版本号，常量池大小，常量池数组，访问控制标记，当前类信息，父类信息，实现的接口个数，实现的接口信息数组，域个数，域信息数组，方法个数，方法信息数组，属性个数，属性信息数组二：查看方法 - javap命令例子：有一个Java类Demo.java1. publicclassDemo2. privateStringstr1;3. privateStringstr2;4. privateintnum1;5. privateintnum2;6. publicstaticfinalStringSTATIC_DATA=helloworld;7. 8. privatevoidsayHello1()9. System.out.println(thisismethod1.);10. 11. privatevoidsayHello2()12. System.out.println(thisismethod2.);13. 14. publicvoidsayHello3()15. System.out.println(thisismethod3.);16. 17. 通过jdk自带的反编译工具命令 javap 可以查看class文件的字节码信息D:javap -verbose Demo Demo.txtDemo.txt：1. CompiledfromDemo.java2. publicclassDemoextendsjava.lang.Object3. SourceFile:Demo.java4. minorversion:05. majorversion:496. 7. Constantpool:8. const#1=class#2;/Demo9. const#2=AscizDemo;10. const#3=class#4;/java/lang/Object11. const#4=Ascizjava/lang/Object;12. const#5=Ascizstr1;13. const#6=AscizLjava/lang/String;14. const#7=Ascizstr2;15. const#8=Asciznum1;16. const#9=AscizI;17. const#10=Asciznum2;18. const#11=AscizSTATIC_DATA;19. const#12=AscizConstantValue;20. const#13=String#14;/helloworld21. const#14=Ascizhelloworld;22. const#15=Asciz;23. const#16=Asciz()V;24. const#17=AscizCode;25. const#18=Method#3.#19;/java/lang/Object.:()V26. const#19=NameAndType#15:#16;/:()V27. const#20=AscizLineNumberTable;28. const#21=AscizLocalVariableTable;29. const#22=Ascizthis;30. const#23=AscizLDemo;31. const#24=AscizsayHello1;32. const#25=Field#26.#28;/java/lang/System.out:Ljava/io/PrintStream;33. const#26=class#27;/java/lang/System34. const#27=Ascizjava/lang/System;35. const#28=NameAndType#29:#30;/out:Ljava/io/PrintStream;36. const#29=Ascizout;37. const#30=AscizLjava/io/PrintStream;38. const#31=String#32;/thisismethod1.39. const#32=Ascizthisismethod1.;40. const#33=Method#34.#36;/java/io/PrintStream.println:(Ljava/lang/String;)V41. const#34=class#35;/java/io/PrintStream42. const#35=Ascizjava/io/PrintStream;43. const#36=NameAndType#37:#38;/println:(Ljava/lang/String;)V44. const#37=Ascizprintln;45. const#38=Asciz(Ljava/lang/String;)V;46. const#39=AscizsayHello2;47. const#40=String#41;/thisismethod2.48. const#41=Ascizthisismethod2.;49. const#42=AscizsayHello3;50. const#43=String#44;/thisismethod3.51. const#44=Ascizthisismethod3.;52. const#45=AscizSourceFile;53. const#46=AscizDemo.java;54. 55. 56. publicstaticfinaljava.lang.StringSTATIC_DATA;57. Constantvalue:Stringhelloworld58. publicDemo();59. Code:60. Stack=1,Locals=1,Args_size=161. 0:aload_062. 1:invokespecial#18;/Methodjava/lang/Object.:()V63. 4:return64. LineNumberTable:65. line2:066. LocalVariableTable:67. StartLengthSlotNameSignature68. 050thisLDemo;69. 70. publicvoidsayHello3();71. Code:72. Stack=2,Locals=1,Args_size=173. 0:getstatic#25;/Fieldjava/lang/System.out:Ljava/io/PrintStream;74. 3:ldc#43;/Stringthisismethod3.75. 5:invokevirtual#33;/Methodjava/io/PrintStream.println:(Ljava/lang/String;)V76. 8:return77. LineNumberTable:78. line17:079. line18:880. LocalVariableTable:81. StartLengthSlotNameSignature82. 090thisLDemo;83. 解析：1.版本号 major version: 49 /java版本 jdk1.6显示的是50， jdk1.5显示的是49，jdk1.4显示的是58 ， 高版本能执行低版本的class文件2.常量池Constant poolMethod：方法Field：字段String：字符串Asciz：签名如由jvm调用，其他是不能够去调用它的NameAndType：变量名的类型Class：类通过字节码，我们可以看到Demo类 继承于java.lang.Object，如果类中没有显式声明构造函数的话，编译器会插入一个缺省无参的构造函数(构造函数在JVM级别是显示成的普通函数)。三：检测代码的效率问题学习Java的过程中，都会了解到字符串合并时要用到StringBuffer 来代替String，那下面就来通过Java字节码来验证两种方式的效率性。例子：一个Java类 TestString.java1. publicclassTestString2. publicStringtestString(Stringstr1,Stringstr2)3. returnstr1+str2;4. 5. publicStringtestStringBuffer(StringBuffersb,Stringstr)6. returnsb.append(str).toString();7. 8. 9. javap c TestString 后字节码信息：1. CompiledfromTestString.java2. publicclassTestStringextendsjava.lang.Object3. publicTestString();4. Code:5. 0:aload_06. 1:invokespecial#8;/Methodjava/lang/Object.:()V7. 4:return8. 9. publicjava.lang.StringtestString(java.lang.String,java.lang.String);10. Code:11. 0:new#16;/classjava/lang/StringBuilder12. 3:dup13. 4:aload_114. 5:invokestatic#18;/Methodjava/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;15. 8:invokespecial#24;/Methodjava/lang/StringBuilder.:(Ljava/lang/String;)V16. 11:aload_217. 12:invokevirtual#27;/Methodjava/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;18. 15:invokevirtual#31;/Methodjava/lang/StringBuilder.toString:()Ljava/lang/String;19. 18:areturn20. 21. publicjava.lang.StringtestStringBuffer(java.lang.StringBuffer,java.lang.String);22. Code:23. 0:aload_124. 1:aload_225. 2:invokevirtual#40;/Methodjava/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;26. 5:invokevirtual#45;/Methodjava/lang/StringBuffer.toString:()Ljava/lang/String;27. 8:areturn28. 从上面编译后的字节码信息可以看出来，方法testString 调用了五个方法：new 、invokestatic 、invokespecial 和两个invokevirtual ; 而testStringBuffer 方法只调用了两个invokevirtual 方法