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# 用Java实现JVM第六章《类和对象》

作者：小傅哥
博客：https://bugstack.cn (opens new window)

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# 案例介绍

本案例通过java代码实现jvm规范中指令集和解释器，完成后就可以开始执行1到100的加和计算。

Java虚拟机顾名思义，就是一台虚拟的机器，而字节码（bytecode）就是运行在这台虚拟机器上的机器码。我们已经知道，每一个类或者接口都会被Java编译器编译成一个class文件，类或接口的方法信息就放在class文件的method_info结构中。如果方法不是抽象的，也不是本地方法，方法的Java代码就会被编译器编译成字节码（即使方法是空的，编译器也会生成一条return语句），存在method_info结构的Code属性中。

# 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
    1. 配置位置：Run/Debug Configurations -> program arguments
    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-06\target\test-classes\org\itstack\demo\test\HelloWorld

# 代码示例

itstack-demo-jvm-06
├── 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   
    │             ├── classpath
    │             │   ├── base
    │             │   ├── comparisons
    │             │   ├── constants
    │             │   ├── control
    │             │   ├── conversions
    │             │   ├── extended
    │             │   ├── loads
    │             │   ├── math
    │             │   │   ├── add
    │             │   │   ├── and
    │             │   │   ├── div
    │             │   │   ├── iinc
    │             │   │   ├── mul
    │             │   │   ├── neg
    │             │   │   ├── or
    │             │   │   ├── rem
    │             │   │   ├── sh
    │             │   │   ├── sub
    │             │   │   └── xor
    │             │   ├── stack
    │             │   ├── store
    │             │   └── Factory	
    │             ├── rtda
    │             │   ├── heap
    │             │   │   ├── constantpool
    │             │   │   ├── methodarea
    │             │   │   │   ├── Class.java 	
    │             │   │   │   ├── ClassMember.java	
    │             │   │   │   ├── Field.java	
    │             │   │   │   ├── Method.java	
    │             │   │   │   ├── Object.java	
    │             │   │   │   └── Slots.java		
    │             │   │   └── ClassLoader.java	
    │             │   ├── Frame.java
    │             │   ├── JvmStack.java
    │             │   ├── LocalVars.java
    │             │   ├── OperandStack.java
    │             │   ├── Slot.java	
    │             │   └── Thread.java
    │             ├── Cmd.java
    │             ├── Interpret.java	
    │             └── Main.java
    └── test
         └── java
             └── org.itstack.demo.test
                 └── HelloWorld.java
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Class.java

public class Class {

    public int accessFlags;
    public String name;
    public String superClassName;
    public String[] interfaceNames;
    public RunTimeConstantPool runTimeConstantPool;
    public Field[] fields;
    public Method[] methods;
    public ClassLoader loader;
    public Class superClass;
    public Class[] interfaces;
    public int instanceSlotCount;
    public int staticSlotCount;
    public Slots staticVars;

    public Class(ClassFile classFile) {
        this.accessFlags = classFile.accessFlags();
        this.name = classFile.className();
        this.superClassName = classFile.superClassName();
        this.interfaceNames = classFile.interfaceNames();
        this.runTimeConstantPool = new RunTimeConstantPool(this, classFile.constantPool());
        this.fields = new Field().newFields(this, classFile.fields());
        this.methods = new Method().newMethods(this, classFile.methods());
    }

    public boolean isPublic() {
        return 0 != (this.accessFlags & AccessFlags.ACC_PUBLIC);
    }

    public boolean isFinal() {
        return 0 != (this.accessFlags & AccessFlags.ACC_FINAL);
    }

    public boolean isSuper() {
        return 0 != (this.accessFlags & AccessFlags.ACC_SUPER);
    }

    public boolean isInterface() {
        return 0 != (this.accessFlags & AccessFlags.ACC_INTERFACE);
    }

    public boolean isAbstract() {
        return 0 != (this.accessFlags & AccessFlags.ACC_ABSTRACT);
    }

    public boolean isSynthetic() {
        return 0 != (this.accessFlags & AccessFlags.ACC_SYNTHETIC);
    }

    public boolean isAnnotation() {
        return 0 != (this.accessFlags & AccessFlags.ACC_ANNOTATION);
    }

    public boolean isEnum() {
        return 0 != (this.accessFlags & AccessFlags.ACC_ENUM);
    }

    public RunTimeConstantPool constantPool() {
        return this.runTimeConstantPool;
    }

    public Slots staticVars() {
        return this.staticVars;
    }

    public boolean isAccessibleTo(Class other) {
        return this.isPublic() || this.getPackageName().equals(other.getPackageName());
    }

    public String getPackageName() {
        int i = this.name.lastIndexOf("/");
        if (i >= 0) return this.name;
        return "";
    }

    public Method getMainMethod() {
        return this.getStaticMethod("main", "([Ljava/lang/String;)V");
    }

    private Method getStaticMethod(String name, String descriptor) {
        for (Method method : this.methods) {
            if (method.name.equals(name) && method.descriptor.equals(descriptor)) {
                return method;
            }
        }
        return null;
    }

    public Object newObject() {
        return new Object(this);
    }

    public boolean isAssignableFrom(Class other) {
        if (this == other) return true;
        if (!other.isInterface()) {
            return this.isSubClassOf(other);
        } else {
            return this.isImplements(other);
        }
    }

    public boolean isSubClassOf(Class other) {
        for (Class c = this.superClass; c != null; c = c.superClass) {
            if (c == other) {
                return true;
            }
        }
        return false;
    }

    private boolean isImplements(Class other) {

        for (Class c = this; c != null; c = c.superClass) {
            for (Class clazz : c.interfaces) {
                if (clazz == other || clazz.isSubInterfaceOf(other)) {
                    return true;
                }
            }
        }
        return false;

    }

    public boolean isSubInterfaceOf(Class iface) {
        for (Class superInterface : this.interfaces) {
            if (superInterface == iface || superInterface.isSubInterfaceOf(iface)) {
                return true;
            }
        }
        return false;
    }

}
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ClassMember.java

public class ClassMember {

    public int accessFlags;
    public String name;
    public String descriptor;
    public Class clazz;

    public void copyMemberInfo(MemberInfo memberInfo) {
        this.accessFlags = memberInfo.accessFlags();
        this.name = memberInfo.name();
        this.descriptor = memberInfo.descriptor();
    }

    public boolean isPublic() {
        return 0 != (this.accessFlags & AccessFlags.ACC_PUBLIC);
    }

    public boolean isPrivate() {
        return 0 != (this.accessFlags & AccessFlags.ACC_PRIVATE);
    }

    public boolean isProtected() {
        return 0 != (this.accessFlags & AccessFlags.ACC_PROTECTED);
    }

    public boolean isStatic() {
        return 0 != (this.accessFlags & AccessFlags.ACC_STATIC);
    }

    public boolean isFinal() {
        return 0 != (this.accessFlags & AccessFlags.ACC_FINAL);
    }

    public boolean isSynthetic() {
        return 0 != (this.accessFlags & AccessFlags.ACC_SYNTHETIC);
    }

    public String name() {
        return this.name;
    }

    public String descriptor() {
        return this.descriptor;
    }

    public Class clazz() {
        return this.clazz;
    }

    public boolean isAccessibleTo(Class d) {
        if (this.isPublic()) {
            return true;
        }
        Class c = this.clazz;
        if (this.isProtected()) {
            return d == c || c.getPackageName().equals(d.getPackageName());
        }
        if (!this.isPrivate()) {
            return c.getPackageName().equals(d.getPackageName());
        }
        return d == c;
    }

}
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Field.java

package org.itstack.demo.jvm.rtda.heap.methodarea;

import org.itstack.demo.jvm.classfile.MemberInfo;
import org.itstack.demo.jvm.classfile.attributes.impl.ConstantValueAttribute;
import org.itstack.demo.jvm.rtda.heap.constantpool.AccessFlags;

public class Field extends ClassMember {

    public int constValueIndex;
    public int slotId;

    public Field[] newFields(Class clazz, MemberInfo[] cfFields) {
        Field[] fields = new Field[cfFields.length];
        for (int i = 0; i < cfFields.length; i++) {
            fields[i] = new Field();
            fields[i].clazz = clazz;
            fields[i].copyMemberInfo(cfFields[i]);
            fields[i].copyAttributes(cfFields[i]);
        }
        return fields;
    }

    public void copyAttributes(MemberInfo cfField) {
        ConstantValueAttribute valAttr = cfField.ConstantValueAttribute();
        if (null != valAttr) {
            this.constValueIndex = valAttr.constantValueIdx();
        }
    }

    public boolean isVolatile() {
        return 0 != (this.accessFlags & AccessFlags.ACC_VOLATILE);
    }

    public boolean isTransient() {
        return 0 != (this.accessFlags & AccessFlags.ACC_TRANSIENT);
    }

    public boolean isEnum() {
        return 0 != (this.accessFlags & AccessFlags.ACC_ENUM);
    }

    public int constValueIndex() {
        return this.constValueIndex;
    }

    public int slotId() {
        return this.slotId;
    }

    public boolean isLongOrDouble() {
        return this.descriptor.equals("J") || this.descriptor.equals("D");
    }


}
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Method.java

package org.itstack.demo.jvm.rtda.heap.methodarea;

import org.itstack.demo.jvm.classfile.MemberInfo;
import org.itstack.demo.jvm.classfile.attributes.impl.CodeAttribute;
import org.itstack.demo.jvm.rtda.heap.constantpool.AccessFlags;

public class Method extends ClassMember {

    public int maxStack;
    public int maxLocals;
    public byte[] code;

    public Method[] newMethods(Class clazz, MemberInfo[] cfMethods) {
        Method[] methods = new Method[cfMethods.length];
        for (int i = 0; i < cfMethods.length; i++) {
            methods[i] = new Method();
            methods[i].clazz = clazz;
            methods[i].copyMemberInfo(cfMethods[i]);
            methods[i].copyAttributes(cfMethods[i]);
        }
        return methods;
    }

    public void copyAttributes(MemberInfo cfMethod) {
        CodeAttribute codeAttr = cfMethod.codeAttribute();
        if (null != codeAttr) {
            this.maxStack = codeAttr.maxStack();
            this.maxLocals = codeAttr.maxLocals();
            this.code = codeAttr.data();
        }
    }

    public boolean isSynchronized() {
        return 0 != (this.accessFlags & AccessFlags.ACC_SYNCHRONIZED);
    }

    public boolean isBridge() {
        return 0 != (this.accessFlags & AccessFlags.ACC_BRIDGE);
    }

    public boolean isVarargs() {
        return 0 != (this.accessFlags & AccessFlags.ACC_VARARGS);
    }

    public boolean isNative() {
        return 0 != (this.accessFlags & AccessFlags.ACC_NATIVE);
    }

    public boolean isAbstract() {
        return 0 != (this.accessFlags & AccessFlags.ACC_ABSTRACT);
    }

    public boolean isStrict() {
        return 0 != (this.accessFlags & AccessFlags.ACC_STRICT);
    }

    public int maxStack() {
        return this.maxStack;
    }

    public int maxLocals() {
        return this.maxLocals;
    }

    public byte[] code() {
        return this.code;
    }

}
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Object.java

package org.itstack.demo.jvm.rtda.heap.methodarea;

public class Object {

    Class clazz;
    Slots fields;

    public Object(Class clazz){
        this.clazz = clazz;
        this.fields = new Slots(clazz.instanceSlotCount);
    }

    public Class clazz(){
        return this.clazz;
    }

    public Slots fields(){
        return this.fields;
    }

    public boolean isInstanceOf(Class clazz){
        return clazz.isAssignableFrom(this.clazz);
    }

}
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Slots.java

package org.itstack.demo.jvm.rtda.heap.methodarea;

import org.itstack.demo.jvm.rtda.Slot;

public class Slots {

    private Slot[] slots;

    public Slots(int slotCount) {
        if (slotCount > 0) {
            slots = new Slot[slotCount];
            for (int i = 0; i < slotCount; i++) {
                slots[i] = new Slot();
            }
        }
    }

    public void setInt(int idx, int val) {
        this.slots[idx].num = val;
    }

    public int getInt(int idx) {
        return this.slots[idx].num;
    }

    public void setFloat(int idx, float val) {
        this.slots[idx].num = (int) val;
    }

    public float getFloat(int idx) {
        return this.slots[idx].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) {
        this.setLong(idx, (long) val);
    }

    public Double getDouble(int idx) {
        return (double) this.getLong(idx);
    }

    public void setRef(int idx, Object ref) {
        this.slots[idx].ref = ref;
    }

    public Object getRef(int idx){
        return this.slots[idx].ref;
    }

}
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# 测试结果 {可以看到已经输出；5050}

"C:\Program Files\Java\jdk1.8.0_161\bin\java.exe" "-javaagent:D:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2018.3.1\lib\idea_rt.jar=61887:D:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2018.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_161\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_161\jre\lib\rt.jar;E:\itstack\git\istack-demo\itstack-demo-jvm\itstack-demo-jvm-05\target\classes;D:\Program Files (x86)\apache-maven-2.2.1\repository\com\beust\jcommander\1.72\jcommander-1.72.jar;D:\Program Files (x86)\apache-maven-2.2.1\repository\org\projectlombok\lombok\1.18.0\lombok-1.18.0.jar;D:\Program Files (x86)\apache-maven-2.2.1\repository\com\alibaba\fastjson\1.2.40\fastjson-1.2.40.jar" org.itstack.demo.jvm.Main -Xjre "C:\Program Files\Java\jdk1.8.0_161\jre" E:\itstack\git\istack-demo\itstack-demo-jvm\itstack-demo-jvm-05\target\test-classes\org\itstack\demo\test\HelloWorld
classpath:org.itstack.demo.jvm.classpath.Classpath@4bf558aa class:E:\itstack\git\istack-demo\itstack-demo-jvm\itstack-demo-jvm-05\target\test-classes\org\itstack\demo\test\HelloWorld args:null
寄存器(指令)：0x03 -> ICONST_0 => 局部变量表：[{"num":0},{"num":0}] 操作数栈：[{"num":0},{"num":0}]
寄存器(指令)：0x3c -> ISTORE_1 => 局部变量表：[{"num":0},{"num":0}] 操作数栈：[{"num":0},{"num":0}]
寄存器(指令)：0x04 -> ICONST_1 => 局部变量表：[{"num":0},{"num":0}] 操作数栈：[{"num":0},{"num":0}]
寄存器(指令)：0x3d -> ISTORE_2 => 局部变量表：[{"num":1},{"num":0}] 操作数栈：[{"num":1},{"num":0}]
寄存器(指令)：0x1c -> ILOAD_2 => 局部变量表：[{"num":1},{"num":0}] 操作数栈：[{"num":1},{"num":0}]
寄存器(指令)：0x10 -> BIPUSH => 局部变量表：[{"num":1},{"num":0}] 操作数栈：[{"num":1},{"num":0}]
寄存器(指令)：0xa3 -> IF_ICMPGT => 局部变量表：[{"num":1},{"num":100}] 操作数栈：[{"num":1},{"num":100}]
寄存器(指令)：0x1b -> ILOAD_1 => 局部变量表：[{"num":1},{"num":100}] 操作数栈：[{"num":1},{"num":100}]
寄存器(指令)：0x1c -> ILOAD_2 => 局部变量表：[{"num":0},{"num":100}] 操作数栈：[{"num":0},{"num":100}]

... ...

寄存器(指令)：0x60 -> IADD => 局部变量表：[{"num":4950},{"num":100}] 操作数栈：[{"num":4950},{"num":100}]
寄存器(指令)：0x3c -> ISTORE_1 => 局部变量表：[{"num":5050},{"num":100}] 操作数栈：[{"num":5050},{"num":100}]
寄存器(指令)：0x84 -> IINC => 局部变量表：[{"num":5050},{"num":100}] 操作数栈：[{"num":5050},{"num":100}]
寄存器(指令)：0xa7 -> GOTO => 局部变量表：[{"num":5050},{"num":100}] 操作数栈：[{"num":5050},{"num":100}]
寄存器(指令)：0x1c -> ILOAD_2 => 局部变量表：[{"num":5050},{"num":100}] 操作数栈：[{"num":5050},{"num":100}]
寄存器(指令)：0x10 -> BIPUSH => 局部变量表：[{"num":101},{"num":100}] 操作数栈：[{"num":101},{"num":100}]
寄存器(指令)：0xa3 -> IF_ICMPGT => 局部变量表：[{"num":101},{"num":100}] 操作数栈：[{"num":101},{"num":100}]
5050
寄存器(指令)：0xb1 -> RETURN => 局部变量表：[{"num":101},{"num":5050}] 操作数栈：[{"num":101},{"num":5050}]
Exception in thread "main" java.lang.RuntimeException: jvm stack is empty!
	at org.itstack.demo.jvm.rtda.JvmStack.pop(JvmStack.java:33)
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