案例介绍
本案例初步实现运行时数据区里；线程、Java虚拟机栈、帧、操作数栈、局部变量表。
在运行Java程序时，Java虚拟机需要使用内存来存放各种各样的数据。Java虚拟机规范把这些内存区域叫作运行时数据区。运行时数据区可以分为两类：一类是多线程共享的，另一类则是线程私有的。多线程共享的运行时数据区需要在Java虚拟机启动时创建好在Java虚拟机推出时销毁。线程私有的运行时数据区则在创建线程时才创建，线程退出时销毁。
线程私有的运行时数据区用于辅助执行Java字节码。每个线程都有自己的pc寄存器（Program Counter）和Java虚拟机栈（JVM Stack）。Java虚拟机栈又由栈帧（Stack Frame，后面简称帧）构成，帧中保存方法执行的状态，包括局部变量表（Local Variable）和操作数栈（Operand Stack）等。在任一时刻，某一线程肯定是在执行某个方法。这个方法叫作该线程的当前方法；执行该方法的帧叫作线程的当前帧；声明该方法的类叫作当前类。如果当前方法是Java方法，则pc寄存器中存放当前正在执行的Java虚拟机指令的地址，否则，当前方法是本地方法，pc寄存器中的值没有明确定义。
Run-Time Data Area ├── Thread │ └── pc │ └── Jvm Stack │ └── Frame │ ├── Local Variable │ └── Operand Stack └── Heap ├── Method Area │ └── Class │ ├── Run-Time │ └── Constant Pool └── Object
环境准备
1、jdk 1.8.0
2、IntelliJ IDEA Community Edition 2018.3.1 x64
配置信息
1、调试配置
2.1、配置位置：Run/Debug Configurations -> program arguments
2.2、配置内容：-Xjre "C:\Program Files\Java\jdk1.8.0_161\jre" E:\itstack\git\istack-demo\itstack-demo-jvm\itstack-demo-jvm-04\target\test-classes\org\itstack\demo\test\HelloWorld
代码示例
itstack-demo-jvm-04 ├── pom.xml └── src └── main │ └── java │ └── org.itstack.demo.jvm │ ├── classfile │ │ ├── attributes {BootstrapMethods/Code/ConstantValue...} │ │ ├── constantpool {CONSTANT_TAG_CLASS/CONSTANT_TAG_FIELDREF/CONSTANT_TAG_METHODREF...} │ │ ├── ClassFile.java │ │ ├── ClassReader.java │ │ └── MemberInfo.java │ ├── classpath │ │ ├── impl │ │ │ ├── CompositeEntry.java │ │ │ ├── DirEntry.java │ │ │ ├── WildcardEntry.java │ │ │ └── ZipEntry.java │ │ ├── Classpath.java │ │ └── Entry.java │ ├── rtda │ │ ├── Frame.java │ │ ├── JvmStack.java │ │ ├── LocalVars.java │ │ ├── OperandStack.java │ │ ├── Slot.java │ │ └── Thread.java │ ├── Cmd.java │ └── Main.java └── test └── java └── org.itstack.demo.test └── HelloWorld.java
Frame.java
package org.itstack.demo.jvm.rtda; /** * http://www.itstack.org * create by fuzhengwei on 2019/4/26 * 栈帧 */ public class Frame { //stack is implemented as linked list Frame lower; //局部变量表 private LocalVars localVars; //操作数栈 private OperandStack operandStack; public Frame(int maxLocals, int maxStack) { this.localVars = new LocalVars(maxLocals); this.operandStack = new OperandStack(maxStack); } public LocalVars localVars(){ return localVars; } public OperandStack operandStack(){ return operandStack; } }
JvmStack.java
package org.itstack.demo.jvm.rtda; /** * http://www.itstack.org * create by fuzhengwei on 2019/4/26 * 虚拟机栈 */ public class JvmStack { private int maxSize; private int size; private Frame _top; public JvmStack(int maxSize) { this.maxSize = maxSize; } public void push(Frame frame) { if (this.size > this.maxSize) { throw new *Error(); } if (this._top != null) { frame.lower = this._top; } this._top = frame; this.size++; } public Frame pop() { if (this._top == null) { throw new RuntimeException("jvm stack is empty!"); } Frame top = this._top; this._top = top.lower; top.lower = null; this.size--; return top; } public Frame top(){ if (this._top == null){ throw new RuntimeException("jvm stack is empty!"); } return this._top; } }
LocalVars.java
package org.itstack.demo.jvm.rtda; /** * http://www.itstack.org * create by fuzhengwei on 2019/4/26 * 局部变量表 */ public class LocalVars { private Slot[] slots; public LocalVars(int maxLocals) { if (maxLocals > 0) { slots = new Slot[maxLocals]; for (int i = 0; i < maxLocals; i++) { slots[i] = new Slot(); } } } public void setInt(int idx, int val) { this.slots[idx].num = val; } public int getInt(int idx) { return slots[idx].num; } public void setFloat(int idx, float val) { this.slots[idx].num = (Float.valueOf(val)).intValue(); } public Float getFloat(int idx) { int num = this.slots[idx].num; return (float) num; } public void setLong(int idx, long val) { this.slots[idx].num = (int) val; this.slots[idx + 1].num = (int) (val >> 32); } public Long getLong(int idx) { int low = this.slots[idx].num; int high = this.slots[idx + 1].num; return ((long) high << 32) | (long) low; } public void setDouble(int idx, double val) { setLong(idx, (long) val); } public Double getDouble(int idx) { return Double.valueOf(getLong(idx)); } public void setRef(int idx, Object ref) { slots[idx].ref = ref; } public Object getRef(int idx) { return slots[idx].ref; } }
OperandStack.java
package org.itstack.demo.jvm.rtda; /** * http://www.itstack.org * create by fuzhengwei on 2019/4/26 * 操作数栈 */ public class OperandStack { private int size = 0; private Slot[] slots; public OperandStack(int maxStack) { if (maxStack > 0) { slots = new Slot[maxStack]; for (int i = 0; i < maxStack; i++) { slots[i] = new Slot(); } } } public void pushInt(int val) { slots[size].num = val; size++; } public int popInt(){ size --; return slots[size].num; } public void pushRef(Object ref){ slots[size].ref = ref; size++; } public Object popRef(){ size --; Object ref = slots[size].ref; slots[size].ref = null; return ref; } }
Slot.java
package org.itstack.demo.jvm.rtda; /** * http://www.itstack.org * create by fuzhengwei on 2019/4/26 * 数据槽 */ public class Slot { int num; Object ref; }
Thread.java
package org.itstack.demo.jvm.rtda; /** * http://www.itstack.org * create by fuzhengwei on 2019/4/26 * 线程 */ public class Thread { //Program Counter 寄存器 private int pc; //虚拟机栈 private JvmStack stack; public Thread(){ this.stack = new JvmStack(1024); } public int pc(){ return this.pc; } public void setPC(int pc){ this.pc = pc; } public void pushFrame(Frame frame){ this.stack.push(frame); } public Frame popFrame(){ return this.stack.pop(); } public Frame currentFrame(){ return this.stack.top(); } }
测试结果
100 -100 null -100