一、源文件

package cn.itcast.jvm.t3.bytecode;/**
* 演示 字节码指令 和 操作数栈、常量池的关系
*/
public class Demo3_1 {public static void main(String[] args) {int a = 10;int b = Short.MAX_VALUE + 1;int c = a + b;System.out.println(c);}
}

二、反编译的字节码文件

[root@localhost ~]# javap -v Demo3_1.class
Classfile /root/Demo3_1.classLast modified Jul 7, 2019; size 665 bytesMD5 checksum a2c29a22421e218d4924d31e6990cfc5Compiled from "Demo3_1.java"
public class cn.itcast.jvm.t3.bytecode.Demo3_1minor version: 0major version: 52flags: ACC_PUBLIC, ACC_SUPER
Constant pool:#1 = Methodref 			#7.#26 			// java/lang/Object."":()V#2 = Class 				#27 			// java/lang/Short#3 = Integer 32768#4 = Fieldref 			#28.#29 		//java/lang/System.out:Ljava/io/PrintStream;#5 = Methodref 			#30.#31 		// java/io/PrintStream.println:(I)V#6 = Class 				#32 			// cn/itcast/jvm/t3/bytecode/Demo3_1#7 = Class 				#33 			// java/lang/Object#8 = Utf8 				#9 = Utf8 				()V#10 = Utf8 				Code#11 = Utf8 				LineNumberTable#12 = Utf8 				LocalVariableTable#13 = Utf8 				this#14 = Utf8 				Lcn/itcast/jvm/t3/bytecode/Demo3_1;#15 = Utf8 				main#16 = Utf8 				([Ljava/lang/String;)V#17 = Utf8 				args#18 = Utf8 				[Ljava/lang/String;#19 = Utf8 				a#20 = Utf8 				I#21 = Utf8 				b#22 = Utf8 				c#23 = Utf8 				MethodParameters#24 = Utf8 				SourceFile#25 = Utf8 				Demo3_1.java#26 = NameAndType 		#8:#9 			// "":()V#27 = Utf8 				java/lang/Short#28 = Class 			#34 			// java/lang/System#29 = NameAndType 		#35:#36 		// out:Ljava/io/PrintStream;#30 = Class 			#37 			// java/io/PrintStream#31 = NameAndType 		#38:#39 		// println:(I)V#32 = Utf8 				cn/itcast/jvm/t3/bytecode/Demo3_1#33 = Utf8 				java/lang/Object#34 = Utf8 				java/lang/System#35 = Utf8 				out#36 = Utf8 				Ljava/io/PrintStream;#37 = Utf8 				java/io/PrintStream#38 = Utf8 				println#39 = Utf8 				(I)V
{public cn.itcast.jvm.t3.bytecode.Demo3_1();descriptor: ()Vflags: ACC_PUBLICCode:stack=1, locals=1, args_size=10: aload_01: invokespecial #1 		// Method java/lang/Object."":()V4: returnLineNumberTable:line 6: 0LocalVariableTable:Start 		Length 		Slot 	Name 	Signature0 			 5 		   0 	this 	Lcn/itcast/jvm/t3/bytecode/Demo3_1;public static void main(java.lang.String[]);descriptor: ([Ljava/lang/String;)Vflags: ACC_PUBLIC, ACC_STATICCode:stack=2, locals=4, args_size=10: bipush 102: istore_13: ldc #3 					// int 327685: istore_26: iload_17: iload_28: iadd9: istore_310: getstatic #4 			// Field java/lang/System.out:Ljava/io/PrintStream;13: iload_314: invokevirtual #5 		// Method java/io/PrintStream.println:(I)V17: returnLineNumberTable:line 8: 0line 9: 3line 10: 6line 11: 10line 12: 17LocalVariableTable:Start 		Length 		Slot 	Name 	Signature0 			18 		   0 	args 	[Ljava/lang/String;3 			15 		   1 	   a 	I6 			12 		   2 	   b 	I10 			 8 		   3 	   c 	IMethodParameters:Name 		Flagsargs
}

三、常量池载入运行时常量池

四、方法字节码载入方法区

五、为main线程分配栈帧内存

（stack=2，locals=4）

六、int a=10;

1）bipush 10

将一个 byte 压入操作数栈（其长度会补齐 4 个字节），类似的指令还有：

• sipush：将一个 short 压入操作数栈（其长度会补齐 4 个字节）
• ldc：将一个 int 压入操作数栈
• ldc2_w：将一个 long 压入操作数栈（分两次压入，因为 long 是 8 个字节）
• 这里小的数字都是和字节码指令存在一起，超过 short 范围的数字存入了常量池

2）istore_1

将操作数栈顶数据弹出，存入局部变量表的 slot 1

七、int b=Short.MAX_VALUE+1；

3）ldc #3

从常量池加载 #3 数据到操作数栈

注意：Short.MAX_VALUE 是 32767，所以 32768 = Short.MAX_VALUE + 1 实际是在编译期间计算 好的

4）istore_2

八、int c=a+b;

5）iload_1

6）iload_2

7）iadd

8）istore_3

九、System.out.println©

9）getstatic #4

10）iload_3

11）invokevirtual #5

找到常量池 #5 项

定位到方法区 java/io/PrintStream.println:(I)V 方法

生成新的栈帧（分配 locals、stack等）

传递参数，执行新栈帧中的字节码

执行完毕，弹出栈帧

清除 main 操作数栈内容

十、返回

12）return

完成 main 方法调用，弹出 main 栈帧

程序结束