-
条件判断指令分析 || JVM类加载与字节码技术
条件判断指令
指令集合
- byte，short，char 都会按 int 比较，因为操作数栈都是 4 字节
- goto 用来进行跳转到指定行号的字节码
源码
package cn.knightzz.jvm.bytecode; /** * @author 王天赐 * @title: ConditionalJudgmentInstruction * @projectName hm-jvm-codes * @description: 条件判断指令 * @website http://knightzz.cn/ * @github https://github.com/knightzz1998 * @create: 2022-10-19 18:53 */ public class ConditionalJudgmentInstruction { public static void main(String[] args) { int a = 0; if (a == 0) { a = 10; } else { a = 20; } } }- 1
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字节码
$ javap -v ConditionalJudgmentInstruction.class Classfile /F:/JavaCode/hm-jvm-codes/jvm-chapter-03/target/classes/cn/knightzz/jvm/bytecode/ConditionalJudgmentInstruction.class Last modified 2022-10-19; size 561 bytes MD5 checksum 4cb9364c4df2604d19885ce96894b9d9 Compiled from "ConditionalJudgmentInstruction.java" public class cn.knightzz.jvm.bytecode.ConditionalJudgmentInstruction minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #3.#20 // java/lang/Object."":()V #2 = Class #21 // cn/knightzz/jvm/bytecode/ConditionalJudgmentInstruction #3 = Class #22 // java/lang/Object #4 = Utf8 <init> #5 = Utf8 ()V #6 = Utf8 Code #7 = Utf8 LineNumberTable #8 = Utf8 LocalVariableTable #9 = Utf8 this #10 = Utf8 Lcn/knightzz/jvm/bytecode/ConditionalJudgmentInstruction; #11 = Utf8 main #12 = Utf8 ([Ljava/lang/String;)V #13 = Utf8 args #14 = Utf8 [Ljava/lang/String; #15 = Utf8 a #16 = Utf8 I #17 = Utf8 StackMapTable #18 = Utf8 SourceFile #19 = Utf8 ConditionalJudgmentInstruction.java #20 = NameAndType #4:#5 // "":()V #21 = Utf8 cn/knightzz/jvm/bytecode/ConditionalJudgmentInstruction #22 = Utf8 java/lang/Object { public cn.knightzz.jvm.bytecode.ConditionalJudgmentInstruction(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: return LineNumberTable: line 12: 0 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this Lcn/knightzz/jvm/bytecode/ConditionalJudgmentInstruction; public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=1, locals=2, args_size=1 0: iconst_0 1: istore_1 2: iload_1 3: ifne 12 6: bipush 10 8: istore_1 9: goto 15 12: bipush 20 14: istore_1 StackMapTable: number_of_entries = 2 frame_type = 252 /* append */ offset_delta = 12 locals = [ int ] frame_type = 2 /* same */ } SourceFile: "ConditionalJudgmentInstruction.java"- 1
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以上比较指令中没有 long，flfloat，double 的比较，那么它们要比较怎么办 ?
参考 : https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-6.html#jvms-6.5.lcmp
核心的字节码 :
0: iconst_0 1: istore_1 2: iload_1 3: ifne 12 6: bipush 10 8: istore_1 9: goto 15 12: bipush 20 14: istore_1 15: return- 1
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- ifne 判断是否不等于 0 , 如果为true 会直接跳转到
12: bipush 20
循环控制指令
源码
package cn.knightzz.jvm.bytecode; /** * @author 王天赐 * @title: CyclicControlInstruction * @projectName hm-jvm-codes * @description: 循环控制指令 * @website http://knightzz.cn/ * @github https://github.com/knightzz1998 * @create: 2022-10-19 19:11 */ public class CyclicControlInstruction { public static void main(String[] args) { int a = 0; while (a < 10) { a++; } } }- 1
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字节码
$ javap -v CyclicControlInstruction.class Classfile /F:/JavaCode/hm-jvm-codes/jvm-chapter-03/target/classes/cn/knightzz/jvm/bytecode/CyclicControlInstruction.class Last modified 2022-10-19; size 538 bytes MD5 checksum 1d37128d3ea691714bb95fcb7f1edec9 Compiled from "CyclicControlInstruction.java" public class cn.knightzz.jvm.bytecode.CyclicControlInstruction minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_SUPER Constant pool: #1 = Methodref #3.#20 // java/lang/Object."":()V #2 = Class #21 // cn/knightzz/jvm/bytecode/CyclicControlInstruction #3 = Class #22 // java/lang/Object #4 = Utf8#5 = Utf8 ()V #6 = Utf8 Code #7 = Utf8 LineNumberTable #8 = Utf8 LocalVariableTable #9 = Utf8 this #10 = Utf8 Lcn/knightzz/jvm/bytecode/CyclicControlInstruction; #11 = Utf8 main #12 = Utf8 ([Ljava/lang/String;)V #13 = Utf8 args #14 = Utf8 [Ljava/lang/String; #15 = Utf8 a #16 = Utf8 I #17 = Utf8 StackMapTable #18 = Utf8 SourceFile #19 = Utf8 CyclicControlInstruction.java #20 = NameAndType #4:#5 // " ":()V #21 = Utf8 cn/knightzz/jvm/bytecode/CyclicControlInstruction #22 = Utf8 java/lang/Object { public cn.knightzz.jvm.bytecode.CyclicControlInstruction(); descriptor: ()V flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: return LineNumberTable: line 12: 0 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this Lcn/knightzz/jvm/bytecode/CyclicControlInstruction; public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=2, args_size=1 0: iconst_0 1: istore_1 2: iload_1 3: bipush 10 5: if_icmpge 14 8: iinc 1, 1 11: goto 2 14: return LineNumberTable: line 16: 0 line 17: 2 line 18: 8 line 21: 14 LocalVariableTable: Start Length Slot Name Signature 0 15 0 args [Ljava/lang/String; 2 13 1 a I StackMapTable: number_of_entries = 2 frame_type = 252 /* append */ offset_delta = 2 locals = [ int ] frame_type = 11 /* same */ } SourceFile: "CyclicControlInstruction.java"- 1
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0: iconst_0 1: istore_1 2: iload_1 3: bipush 10 5: if_icmpge 14 8: iinc 1, 1 11: goto 2 14: return- 1
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- iconst_0 创建一个值为 0 的常量
- istore_1 赋值给 插槽1的变量 a
- iload_1 将a的值压入栈
- bipush 10 将10压入栈
- if_icmpge : 判断两个int是否 >= ==>
a >= 10- 如果为true , 直接跳到 14:return
- 如果为false, iinc 1,1 => 插槽1中的元素a累加1
- goto 2 ==> 2: iload_1
其他的如 do while 和 for 其实都是类似的
练习-判断结果
源码
package cn.knightzz.jvm.bytecode; /** * @author 王天赐 * @title: JudgementResult * @projectName hm-jvm-codes * @description: 判断结果 * @website http://knightzz.cn/ * @github https://github.com/knightzz1998 * @create: 2022-10-19 19:22 */ public class JudgementResult { public static void main(String[] args) { int i = 0; int x = 0; while (i < 10) { x = x++; i++; } System.out.println(x); // 结果是 0 } }- 1
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字节码
// int i = 0 iconst_0 istore_1 // int x = 0 iconst_0 istore_2 iload_1 bipush 10 if_icmpge xxx iload_2 // 将 0 加载进操作数栈 iinc 2,1 // 直接在局部变量表中累加 istore_2 // 将操作数栈中的元素 0 重新赋值到 x(slot_2) iload_1 iinc 1,1 goto xxx getstatic #x invoke xx- 1
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最终的结果为0的原因是因为 :
- iload_2 ==> 将 0 加载进操作数栈
- iinc 2,1 ==> 直接在局部变量表中累加 , 此时 x = 1
- istore_2 // 将操作数栈中的元素 0 重新赋值到 x(slot_2) 此时 x = 0
赋值指令是从操作数栈复制到局部变量表中 , 而累加是 直接在局部变量表中累加
生成的字节码 :
0 iconst_0 1 istore_1 2 iconst_0 3 istore_2 4 iload_1 5 bipush 10 7 if_icmpge 21 (+14) 10 iload_2 11 iinc 2 by 1 14 istore_2 15 iinc 1 by 1 18 goto 4 (-14) 21 getstatic #224 iload_2 25 invokevirtual #3 28 return - 1
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- 原文地址：https://blog.csdn.net/weixin_40040107/article/details/127415737
