什么是字節(jié)碼

Java bytecode是由單個(byte)組成，理論上最多支持256個操作碼(opcode)。實際上Java只用了200多個左右的操作碼，還有一些操作碼留給調(diào)試操作。

根據(jù)操作碼的性質(zhì)主要分為四大類

棧操作指令，包括與局部變量操作指令

程序流程控制指令

對象操作指令，包括方法調(diào)用指令

算數(shù)運算及類型轉(zhuǎn)換指令

生成字節(jié)碼

寫一個最簡單的類源碼如下

package demo.jvm0104;public class HelloByteCode{public static void main(String[] args){HelloByteCode obj=new HelloByteCode();} }

編譯：javac demo/jvm0104/HelloByteCode.java
查看字節(jié)碼：javap -c demo.jvm0104/HelloByteCode

Compiled from "HelloByteCode.java" public class demo.jvm0104.HelloByteCode {public demo.jvm0104.HelloByteCode();Code:0: aload_01: invokespecial #1 // Method java/lang/Object."<init>":()V4: returnpublic static void main(java.lang.String[]);Code:0: new #2 // class demo/jvm0104/HelloByteCode3: dup4: invokespecial #3 // Method "<init>":()V7: astore_18: return }

復(fù)雜點的例子：javap -c -verbose demo.java0104.Hello

Classfile /C:/Users/29328/Desktop/abc/demo/jvm0104/HelloByteCode.classLast modified 2021-3-6; size 301 bytesMD5 checksum 542cb70faf8b2b512a023e1a8e6c1308Compiled from "HelloByteCode.java" public class demo.jvm0104.HelloByteCodeminor version: 0major version: 52flags: ACC_PUBLIC, ACC_SUPER Constant pool:#1 = Methodref #4.#13 // java/lang/Object."<init>":()V#2 = Class #14 // demo/jvm0104/HelloByteCode#3 = Methodref #2.#13 // demo/jvm0104/HelloByteCode."<init>":()V#4 = Class #15 // java/lang/Object#5 = Utf8 <init>#6 = Utf8 ()V#7 = Utf8 Code#8 = Utf8 LineNumberTable#9 = Utf8 main#10 = Utf8 ([Ljava/lang/String;)V#11 = Utf8 SourceFile#12 = Utf8 HelloByteCode.java#13 = NameAndType #5:#6 // "<init>":()V#14 = Utf8 demo/jvm0104/HelloByteCode#15 = Utf8 java/lang/Object {public demo.jvm0104.HelloByteCode();descriptor: ()Vflags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1 // Method java/lang/Object."<init>":()V4: returnLineNumberTable:line 3: 0public static void main(java.lang.String[]);descriptor: ([Ljava/lang/String;)Vflags: ACC_PUBLIC, ACC_STATICCode:stack=2, locals=2, args_size=10: new #2 // class demo/jvm0104/HelloByteCode3: dup4: invokespecial #3 // Method "<init>":()V7: astore_18: returnLineNumberTable:line 5: 0line 6: 8 } SourceFile: "HelloByteCode.java"

字節(jié)碼運行時節(jié)構(gòu)

Jvm是一臺基于棧的獨立計算機。
每個線程都有自己獨立的線程棧(Jvm Stack)，用于存儲自己的棧幀(Frame)。
每次操作Jvm都會創(chuàng)建自己的棧幀，棧幀由操作數(shù)棧，局部變量數(shù)組及Class引用組成。
Class引用指向當前運行時常量中對應(yīng)的Class類。

四則運算例子

編寫MovingAverage.java類

package demo.jvm0104;/** * 移動平均數(shù) */ public class MovingAverage{private int count=0;private double sum=0.0D;public void submit(double value){this.count++;this.sum+=value;}public double getAvg(){if(0==this.count){return sum;}else{return this.sum/this.count;}} }

編譯：javac -encoding utf-8 demo/jvm0104/MovingAverage.java
查看字節(jié)碼：javap -c demo.jvm0104.MovingAverage

Compiled from "MovingAverage.java" public class demo.jvm0104.MovingAverage {public demo.jvm0104.MovingAverage();Code:0: aload_01: invokespecial #1 // Method java/lang/Object."<init>":()V4: aload_05: iconst_06: putfield #2 // Field count:I9: aload_010: dconst_011: putfield #3 // Field sum:D14: returnpublic void submit(double);Code:0: aload_01: dup2: getfield #2 // Field count:I5: iconst_16: iadd7: putfield #2 // Field count:I10: aload_011: dup12: getfield #3 // Field sum:D15: dload_116: dadd17: putfield #3 // Field sum:D20: returnpublic double getAvg();Code:0: iconst_01: aload_02: getfield #2 // Field count:I5: if_icmpne 138: aload_09: getfield #3 // Field sum:D12: dreturn13: aload_014: getfield #3 // Field sum:D17: aload_018: getfield #2 // Field count:I21: i2d22: ddiv23: dreturn }

編寫LocalVariableTest.java類

package demo.jvm0104;public class LocalVariableTest{public static void main(String[] args){MovingAverage ma=new MovingAverage();int num1=1;int num2=2;ma.submit(num1);ma.submit(num2);double avg=ma.getAvg();} }

編譯：javac -encoding utf-8 demo/jvm0104/LocalVariableTest.java
查看字節(jié)碼：javap -c demo.jvm0104.LocalVaiableTest

Compiled from "LocalVariableTest.java" public class demo.jvm0104.LocalVariableTest {public demo.jvm0104.LocalVariableTest();Code:0: aload_01: invokespecial #1 // Method java/lang/Object."<init>":()V4: returnpublic static void main(java.lang.String[]);Code:0: new #2 // class demo/jvm0104/MovingAverage3: dup4: invokespecial #3 // Method demo/jvm0104/MovingAverage."<init>":()V7: astore_18: iconst_19: istore_210: iconst_211: istore_312: aload_113: iload_214: i2d15: invokevirtual #4 // Method demo/jvm0104/MovingAverage.submit:(D)V18: aload_119: iload_320: i2d21: invokevirtual #4 // Method demo/jvm0104/MovingAverage.submit:(D)V24: aload_125: invokevirtual #5 // Method demo/jvm0104/MovingAverage.getAvg:()D28: dstore 430: return

算術(shù)操作與類型轉(zhuǎn)化

add(+)sub(-)mult(*)divide(/)remainder(%)negate(-)

int	iadd	isub	imul	idiv	irem	ineg
long	ladd	lsub	lmul	ldiv	lrem	lneg
float	fadd	fsub	fmul	fdiv	frem	fneg
double	dadd	dsub	dmul	ddiv	drem	dneg

intlongfloatdoublebytecharshort

int	-	i2l	i2f	i2d	i2b	i2c	i2s
long	l2i	-	l2f	l2d	-	-	-
float	f2i	f2l	-	f2d	-	-	-
double	d2i	d2l	d2f	-	-	-	-

總結(jié)

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