深入理解字节码点击这里 

jdk中的bin里面自带有反编译的程序，叫javap.exe,利用他可以从编译生成的.class查看出对应的字节码代码

具体方法如下:

1.javap -verbose ** >> **.txt

例:

随便写个java类Person

class Person{String name;	char sex;int age;Person(){}Person(String name,char sex,int age){this.name=name;this.sex=sex;this.age=age;	}String toString(int a){return(String.valueOf(a));}String toString(char a){return(String.valueOf(a));}
}

编译生成.class后,放到桌面上，shift+右键在当前窗口打开命令行（win7可以，xp不行，就是去.class对应的目录输入命令）输入

javap -verbose Person >> Person.txt

然后在桌面上就看到了Person.txt

内容:

Classfile /C:/Users/linux_v/Desktop/Person.classLast modified Oct 11, 2012; size 589 bytesMD5 checksum 137de4fcce084d25ffb5c93b1708f63aCompiled from "Person.java"
class PersonSourceFile: "Person.java"minor version: 0major version: 51flags: ACC_SUPERConstant pool:#1 = Methodref          #8.#25         //  java/lang/Object."<init>":()V#2 = Fieldref           #7.#26         //  Person.name:Ljava/lang/String;#3 = Fieldref           #7.#27         //  Person.sex:C#4 = Fieldref           #7.#28         //  Person.age:I#5 = Methodref          #29.#30        //  java/lang/String.valueOf:(I)Ljava/lang/String;#6 = Methodref          #29.#31        //  java/lang/String.valueOf:(C)Ljava/lang/String;#7 = Class              #32            //  Person#8 = Class              #33            //  java/lang/Object#9 = Utf8               name#10 = Utf8               Ljava/lang/String;#11 = Utf8               sex#12 = Utf8               C#13 = Utf8               age#14 = Utf8               I#15 = Utf8               <init>#16 = Utf8               ()V#17 = Utf8               Code#18 = Utf8               LineNumberTable#19 = Utf8               (Ljava/lang/String;CI)V#20 = Utf8               toString#21 = Utf8               (I)Ljava/lang/String;#22 = Utf8               (C)Ljava/lang/String;#23 = Utf8               SourceFile#24 = Utf8               Person.java#25 = NameAndType        #15:#16        //  "<init>":()V#26 = NameAndType        #9:#10         //  name:Ljava/lang/String;#27 = NameAndType        #11:#12        //  sex:C#28 = NameAndType        #13:#14        //  age:I#29 = Class              #34            //  java/lang/String#30 = NameAndType        #35:#21        //  valueOf:(I)Ljava/lang/String;#31 = NameAndType        #35:#22        //  valueOf:(C)Ljava/lang/String;#32 = Utf8               Person#33 = Utf8               java/lang/Object#34 = Utf8               java/lang/String#35 = Utf8               valueOf
{java.lang.String name;flags: char sex;flags: int age;flags: Person();flags: Code:stack=1, locals=1, args_size=10: aload_0       1: invokespecial #1                  // Method java/lang/Object."<init>":()V4: return        LineNumberTable:line 5: 0line 6: 4Person(java.lang.String, char, int);flags: Code:stack=2, locals=4, args_size=40: aload_0       1: invokespecial #1                  // Method java/lang/Object."<init>":()V4: aload_0       5: aload_1       6: putfield      #2                  // Field name:Ljava/lang/String;9: aload_0       10: iload_2       11: putfield      #3                  // Field sex:C14: aload_0       15: iload_3       16: putfield      #4                  // Field age:I19: return        LineNumberTable:line 7: 0line 8: 4line 9: 9line 10: 14line 11: 19java.lang.String toString(int);flags: Code:stack=1, locals=2, args_size=20: iload_1       1: invokestatic  #5                  // Method java/lang/String.valueOf:(I)Ljava/lang/String;4: areturn       LineNumberTable:line 13: 0java.lang.String toString(char);flags: Code:stack=1, locals=2, args_size=20: iload_1       1: invokestatic  #6                  // Method java/lang/String.valueOf:(C)Ljava/lang/String;4: areturn       LineNumberTable:line 16: 0
}

看着挺高端的，有时间再慢慢研究，还有另一种命令

2.javap –c ** >> **.txt

上面链接给的测试类，我们确实时常讨论String,StringBuilder,StringBuffer之间的区别，其实看字节码确实能看出一点区别，即使不懂字节码。

TestString.java

public class TestString { public String testString(String str1, String str2){ return str1 + str2; } public String testStringBuffer(StringBuffer sb, String str){ return sb.append(str).toString(); }public String testStringBuilder(StringBuilder sb, String str){return sb.append(str).toString();} 
} 

编译生成字节码后，输入

javap -c TestString >> TestString.txt

生成文本如下:

Compiled from "TestString.java"
public class TestString {public TestString();Code:0: aload_0       1: invokespecial #1                  // Method java/lang/Object."<init>":()V4: return        public java.lang.String testString(java.lang.String, java.lang.String);Code:0: new           #2                  // class java/lang/StringBuilder3: dup           4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V7: aload_1       8: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;11: aload_2       12: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;15: invokevirtual #5                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;18: areturn       public java.lang.String testStringBuffer(java.lang.StringBuffer, java.lang.String);Code:0: aload_1       1: aload_2       2: invokevirtual #6                  // Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;5: invokevirtual #7                  // Method java/lang/StringBuffer.toString:()Ljava/lang/String;8: areturn       
}
Compiled from "TestString.java"
public class TestString {public TestString();Code:0: aload_0       1: invokespecial #1                  // Method java/lang/Object."<init>":()V4: return        public java.lang.String testString(java.lang.String, java.lang.String);Code:0: new           #2                  // class java/lang/StringBuilder3: dup           4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V7: aload_1       8: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;11: aload_2       12: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;15: invokevirtual #5                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;18: areturn       public java.lang.String testStringBuffer(java.lang.StringBuffer, java.lang.String);Code:0: aload_1       1: aload_2       2: invokevirtual #6                  // Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;5: invokevirtual #7                  // Method java/lang/StringBuffer.toString:()Ljava/lang/String;8: areturn       public java.lang.String testStringBuilder(java.lang.StringBuilder, java.lang.String);Code:0: aload_1       1: aload_2       2: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;5: invokevirtual #5                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;8: areturn       
}

可以看到对于定义String类型的字符做字符连接时所执行的方法要多于StringBuffer和StringBuilder,StringBuffer和StringBuilder是一样的，不过StringBuilder是非线程安全的，StringBuffer是线程安全的，所以一般推荐使用StringBuffer，以前看人用java代码测试运行时间来观察执行效率，其实这样通过字节码大体猜测感觉也是挺不错的。