java基础之string字节码初始
---- 待完善............................
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1. java代码
public class Test2 {public void testString() {String a = "hello";String b = "word";String c = a + b;System.out.println(c);}public void testString2() {String c = "";for(int i=0;i<10;i++) {c = c+i;}System.out.println(c);}public void testStringBuffer() {String a = "hello";String b = "word";StringBuffer c = new StringBuffer("");c.append(a).append(b);System.out.println(c);}public void testStringBuffer2() {StringBuffer c = new StringBuffer("");for(int i=0;i<10;i++) {c = c.append(i);}System.out.println(c);}}?
2. 字节码
Compiled from "Test2.java"public class com.java.test.Test2 extends java.lang.Object{public com.java.test.Test2(); Code: 0:aload_0 -- 将第一个引用类型本地变量推送至栈顶 1:invokespecial#9; //Method java/lang/Object."<init>":()V -- 调用超类构造方法，实例初始化方法，私有方法 4:return -- 从当前方法返回voidpublic void testString(); Code: 0:ldc#16; //String hello -- 将int, float或String型常量值从常量池中推送至栈顶 2:astore_1 -- 将栈顶引用型数值存入第二个本地变量 3:ldc#18; //String word 5:astore_2 6:new#20; //class java/lang/StringBuilder -- 创建一个新的对象，并将其引用值压入栈顶。 #20是常量池索引（本类其它和String有关的new这个值都是一样的）。 -- 表示常量池中第20个索引的值。class java/lang/StringBuilder，该指令占用3个字节。 9:dup -- 复制栈顶数值并将复制值压入栈顶 --JVM新创建一个对象后，首先需要调用<init>实例初始化方法， --初始化完成后再会做保存到局部变量或调用方法的处理（类似代码：Object obj = new Object();）， --这样就需要在操作数栈上保留两个相同的对象引用，所以需要dup。 10:aload_1 -- 将第二个引用类型本地变量推送至栈顶 11:invokestatic#26; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String; -- 调用静态方法 String.valueOf 14:invokespecial#29; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V -- 调用超类构造方法，实例初始化方法，私有方法 调用StringBuilder的init方法 17:aload_2 -- 将第三个引用类型本地变量推送至栈顶 18:invokevirtual#33; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; -- 调用实例方法，调用StringBuilder.append 21:invokevirtual#37; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; -- 调用实例方法，调用StringBuilder.toString 24:astore_3 -- 将栈顶引用型 数值存入第四个本地变量 25:getstatic#43; //Field java/lang/System.out:Ljava/io/PrintStream; -- 获取指定类的静态域，并将其值压入栈顶 28:aload_3 -- 将第四个引用类型本地变量推送至栈顶 29:invokevirtual#48; //Method java/io/PrintStream.println:(Ljava/lang/String;)V -- 调用实例方法 println PrintStream.println 32:returnpublic void testString2(); Code: 0:ldc#55; //String -- 将int, float或String型常量值从常量池中推送至栈顶 2:astore_1 3:iconst_0 4:istore_2 5:goto30 -- 无条件跳转 即跳转到 30:iload_2 这里 8:new#20; //class java/lang/StringBuilder 11:dup 12:aload_1 13:invokestatic#26; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String; 16:invokespecial#29; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 19:iload_2 20:invokevirtual#58; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder; 23:invokevirtual#37; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 26:astore_1 27:iinc2, 1 --将指定int型变量增加指定值（i++, i--, i+=2） 30:iload_2 --将第三个int型本地变量推送至栈顶 31:bipush10 --将单字节的常量值(-128~127)推送至栈顶 33:if_icmplt8 --比较栈顶两int型数值大小，当结果小于0时跳转 36:getstatic#43; //Field java/lang/System.out:Ljava/io/PrintStream; 39:aload_1 40:invokevirtual#48; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 43:returnpublic void testStringBuffer(); Code: 0:ldc#16; //String hello 2:astore_1 3:ldc#18; //String word 5:astore_2 6:new#63; //class java/lang/StringBuffer 9:dup 10:ldc#55; //String 12:invokespecial#64; //Method java/lang/StringBuffer."<init>":(Ljava/lang/String;)V 15:astore_3 16:aload_3 17:aload_1 18:invokevirtual#67; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 21:aload_2 22:invokevirtual#67; //Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer; 25:pop --将栈顶数值弹出 (数值不能是long或double类型的) 26:getstatic#43; //Field java/lang/System.out:Ljava/io/PrintStream; 29:aload_3 30:invokevirtual#70; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V 33:returnpublic void testStringBuffer2(); Code: 0:new#63; //class java/lang/StringBuffer 3:dup 4:ldc#55; //String 6:invokespecial#64; //Method java/lang/StringBuffer."<init>":(Ljava/lang/String;)V 9:astore_1 10:iconst_0 11:istore_2 12:goto24 15:aload_1 16:iload_2 17:invokevirtual#75; //Method java/lang/StringBuffer.append:(I)Ljava/lang/StringBuffer; 20:astore_1 21:iinc2, 1 -- iinc index ,const -- The index is an unsigned byte that must be an index into the local variable array of -- the current frame . The const is an immediate signed byte. The local variable -- at index must contain an int. The value const is first sign-extended to an int, and -- then the local variable at index is incremented by that amount. 24:iload_2 25:bipush10 27:if_icmplt15 30:getstatic#43; //Field java/lang/System.out:Ljava/io/PrintStream; 33:aload_1 34:invokevirtual#70; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V 37:return}?
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