思考

编写-编译-运行。java文件编译后生成class文件，jvm是如何加载class文件？

1 准备工作

获取class文件byte[]

 public static byte[] getFileBytes(File file) {
        try (FileInputStream fileInputStream = new FileInputStream(file)) {
            int available = fileInputStream.available();
            byte[] data=new byte[available];
            fileInputStream.read(data,0,available-1);
            return data;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

这里使用jdk的ByteBuffer包装bytes

ByteBuffer data = ByteBuffer.wrap(getFileBytes(file));

因为ByteBuffer没有无符号的读取方法,所以自己实现一下,也可以直接用netty的Bytebuf,里面方法齐全

    // 图方便直接返回int
    private int readUnsignedByte(ByteBuffer data) {
        return data.get() & 0xff;
    }

    // 图方便直接返回int
    private int readUnsignedShort(ByteBuffer data) {
        return data.getShort() & 0xffff;
    }
    
    private long readUnsignedInt(ByteBuffer data) {
        return data.getInt() & 0xffffffffL;
    }

定义class文件结构

参考: https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html

    private static class ClassFileStructure {
        long magic;
        int minorVersion;
        int majorVersion;
        int constantPoolCount;
        ConstantPool[] constantPool;
        int accessFlags;
        int thisClass;
        int superClass;
        int interfacesCount;
        int[] interfaces;
        int fieldsCount;
        FieldInfo[] fields;
        int methodsCount;
        MethodInfo[] methods;
        int attributesCount;
        AttributeInfo[] attributes;
    }

2 开始解析

2.1 magic

    private void magic(ClassFileStructure structure, ByteBuffer data) {
        structure.magic = readUnsignedInt(data);
    }

2.2 minorVersion

    private void minorVersion(ClassFileStructure structure, ByteBuffer data) {
        structure.minorVersion = readUnsignedShort(data);
    }

2.3 majorVersion

    private void majorVersion(ClassFileStructure structure, ByteBuffer data) {
        structure.majorVersion = readUnsignedShort(data);
    }

2.4 constantPoolCount

   private void constantPoolCount(ClassFileStructure structure, ByteBuffer data) {
        structure.constantPoolCount = readUnsignedShort(data);
   }

2.5 ConstantPool[]

ConstantPool不同tag解析方式不同,定义抽象类ConstantPool,子类按规则解析

    private abstract static class ConstantPool {
    int tag;

    public ConstantPool(int tag) {
        this.tag = tag;
    }

    abstract void parse(ByteBuffer data);
}

子类实现ConstantPool
ConstantUtf8:

 private class ConstantUtf8 extends ConstantPool {
    int length;
    byte[] bytes;

    public ConstantUtf8(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.length = readUnsignedShort(data);
        bytes = new byte[this.length];
        for (int i = 0; i < this.length; i++) {
            bytes[i] = (byte) readUnsignedByte(data);
        }
    }
}

ConstantMethodHandle:

 private class ConstantMethodHandle extends ConstantPool {
    short referenceKind;
    int referenceIndex;

    public ConstantMethodHandle(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.referenceKind = (short) readUnsignedByte(data);
        this.referenceIndex = readUnsignedShort(data);
    }
}

ConstantMethodType:

 private class ConstantMethodType extends ConstantPool {
    int descriptorIndex;

    public ConstantMethodType(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.descriptorIndex = readUnsignedShort(data);
    }
}

ConstantClass:

private class ConstantClass extends ConstantPool {
    int nameIndex;

    public ConstantClass(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.nameIndex = readUnsignedShort(data);
    }
}

ConstantClass:

   private class ConstantClass extends ConstantPool {
    int nameIndex;

    public ConstantClass(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.nameIndex = readUnsignedShort(data);
    }
}

ConstantFieldref:

private class ConstantFieldref extends ConstantPool {
    int classIndex;
    int nameAndTypeIndex;

    public ConstantFieldref(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.classIndex = readUnsignedShort(data);
        this.nameAndTypeIndex = readUnsignedShort(data);
    }
}

ConstantMethodref:

private class ConstantMethodref extends ConstantPool {
    int classIndex;
    int nameAndTypeIndex;

    public ConstantMethodref(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.classIndex = readUnsignedShort(data);
        this.nameAndTypeIndex = readUnsignedShort(data);
    }
}

ConstantInterfaceMethodref:

 private class ConstantInterfaceMethodref extends ConstantPool {
    int classIndex;
    int nameAndTypeIndex;

    public ConstantInterfaceMethodref(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.classIndex = readUnsignedShort(data);
        this.nameAndTypeIndex = readUnsignedShort(data);
    }
}

ConstantString:

 private class ConstantString extends ConstantPool {
    int stringIndex;

    public ConstantString(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.stringIndex = readUnsignedShort(data);
    }
}

ConstantInteger:

private class ConstantInteger extends ConstantPool {
    long bytes;

    public ConstantInteger(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.bytes = readUnsignedInt(data);
    }
}

ConstantFloat:

private class ConstantFloat extends ConstantPool {
    long bytes;

    public ConstantFloat(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.bytes = readUnsignedInt(data);
    }
}

ConstantLong:

private class ConstantLong extends ConstantPool {
    long highBytes;
    long lowBytes;

    public ConstantLong(int tag) {
        super(tag);
    }

    @Override
    void parse(ByteBuffer data) {
        this.highBytes = readUnsignedInt(data);
        this.lowBytes = readUnsignedInt(data);
    }
}

ConstantDouble:

   private class ConstantDouble extends ConstantPool {
        long highBytes;
        long lowBytes;
        public ConstantDouble(int tag) {
            super(tag);
        }
        @Override
        void parse(ByteBuffer data) {
            this.highBytes = readUnsignedInt(data);
            this.lowBytes = readUnsignedInt(data);
        }
    }

ConstantNameAndType:

  private class ConstantNameAndType extends ConstantPool {
        int nameIndex;
        int descriptorIndex;
        public ConstantNameAndType(int tag) {
            super(tag);
        }
        @Override
        void parse(ByteBuffer data) {
            this.nameIndex = readUnsignedShort(data);
            this.descriptorIndex = readUnsignedShort(data);
        }
    }

ConstantInvokeDynamic:

  private class ConstantInvokeDynamic extends ConstantPool {
        int bootstrapMethodAttrIndex;
        int nameAndTypeIndex;
        public ConstantInvokeDynamic(int tag) {
            super(tag);
        }
        @Override
        void parse(ByteBuffer data) {
            this.bootstrapMethodAttrIndex = readUnsignedShort(data);
            this.nameAndTypeIndex = readUnsignedShort(data);
        }
    }

以上是所有 constantPool 子类型,现在开始解析

坑：“所有 8 字节常量都占用文件constant_pool表中的两个条目class。 如果
CONSTANT_Long_infoorCONSTANT_Double_info结构是constant_pool表中索引n处的项目，则池中的下一个可用项目位于索引n +2 处。
constant_pool 索引n +1 必须有效但被视为不可用 。 回想起来，让 8 字节常量占用两个常量池条目是一个糟糕的选择。”

 private void constantPool(ClassFileStructure structure, ByteBuffer data) {
        structure.constantPool = new ConstantPool[structure.constantPoolCount - 1];
        for (int i = 0; i < structure.constantPool.length; i++) {
            int type = readUnsignedByte(data);
            int index = i;
            switch (type) {
                case 1: //
                    structure.constantPool[i] = new ConstantUtf8(type);
                    break;
                case 3: //
                    structure.constantPool[i] = new ConstantInteger(type);
                    break;
                case 4:
                    structure.constantPool[i] = new ConstantFloat(type);
                    break;
                case 5:
                    structure.constantPool[i] = new ConstantLong(type);
                    i++;// 占2位
                    break;
                case 6:
                    structure.constantPool[i] = new ConstantDouble(type);
                    i++;// 占2位
                    break;
                case 7: //
                    structure.constantPool[i] = new ConstantClass(type);
                    break;
                case 8: //
                    structure.constantPool[i] = new ConstantString(type);
                    break;
                case 9: //
                    structure.constantPool[i] = new ConstantFieldref(type);
                    break;
                case 10: //
                    structure.constantPool[i] = new ConstantMethodref(type);
                    break;
                case 11: //
                    structure.constantPool[i] = new ConstantInterfaceMethodref(type);
                    break;
                case 12: //
                    structure.constantPool[i] = new ConstantNameAndType(type);
                    break;
                case 15: //
                    structure.constantPool[i] = new ConstantMethodHandle(type);
                    break;
                case 16: //
                    structure.constantPool[i] = new ConstantMethodType(type);
                    break;
                case 18: //
                    structure.constantPool[i] = new ConstantInvokeDynamic(type);
                    break;
                default:
                    throw new ParserException("class file parser exception");
            }
            structure.constantPool[index].parse(data);
        }
    }

2.6 accessFlags

    private void accessFlags(ClassFileStructure structure, ByteBuffer data) {
        structure.accessFlags = readUnsignedShort(data);
    }

2.7 thisClass

    private void thisClass(ClassFileStructure structure, ByteBuffer data) {
        structure.thisClass = readUnsignedShort(data);
    }

2.8 superClass

    private void superClass(ClassFileStructure structure, ByteBuffer data) {
        structure.superClass = readUnsignedShort(data);
    }

2.9 interfacesCount

    private void interfacesCount(ClassFileStructure structure, ByteBuffer data) {
        structure.interfacesCount = readUnsignedShort(data);
    }

2.10 int[] interfaces

    private void interfaces(ClassFileStructure structure, ByteBuffer data) {
        structure.interfaces = new int[structure.interfacesCount];
        for (int i = 0; i < structure.interfacesCount; i++) {
            structure.interfaces[i] = readUnsignedShort(data);
        }
    }

2.11 fieldsCount

    private ClassFile fieldsCount(ClassFileStructure structure, ByteBuffer data) {
        structure.fieldsCount = readUnsignedShort(data);
        return this;
    }

2.12 FieldInfo[] fields

FieldInfo：

 private class FieldInfo {
        int accessFlags;
        int nameIndex;
        int descriptorIndex;
        int attributesCount;
        AttributeInfo[] attributes;

        public FieldInfo parse(ByteBuffer data) {
            this.accessFlags = readUnsignedShort(data);
            this.nameIndex = readUnsignedShort(data);
            this.descriptorIndex = readUnsignedShort(data);
            this.attributesCount = readUnsignedShort(data);
            this.attributes = new AttributeInfo[attributesCount];
            for (int i = 0; i < this.attributesCount; i++) {
                this.attributes[i] = new AttributeInfo().parse(data);
            }
            return this;
        }
    }

AttributeInfo:

  private class AttributeInfo {

        int attributeNameIndex;
        long attributeLength;
        short[] info;

        public AttributeInfo parse(ByteBuffer data) {
            this.attributeNameIndex = readUnsignedShort(data);
            this.attributeLength = readUnsignedInt(data);
            this.info = new short[(int) attributeLength];
            for (int i = 0; i < this.attributeLength; i++) {
                this.info[i] = (short) readUnsignedByte(data);
            }
            return this;
        }
    }

    private void fields(ClassFileStructure structure, ByteBuffer data) {
        structure.fields = new FieldInfo[structure.fieldsCount];
        for (int i = 0; i < structure.fieldsCount; i++) {
            structure.fields[i] = new FieldInfo().parse(data);
        }
    }

2.13 methodsCount

    private ClassFile methodsCount(ClassFileStructure structure, ByteBuffer data) {
        structure.methodsCount = readUnsignedShort(data);
        return this;
    }

2.14 MethodInfo[]
MethodInfo:

 private class MethodInfo {
        int accessFlags;
        int nameIndex;
        int descriptorIndex;
        int attributesCount;
        AttributeInfo[] attributes;

        public MethodInfo parse(ByteBuffer data) {
            this.accessFlags = readUnsignedShort(data);
            this.nameIndex = readUnsignedShort(data);
            this.descriptorIndex = readUnsignedShort(data);
            this.attributesCount = readUnsignedShort(data);
            this.attributes = new AttributeInfo[attributesCount];
            for (int i = 0; i < this.attributesCount; i++) {
                this.attributes[i] = new AttributeInfo().parse(data);
            }
            return this;
        }
    }

    private void methods(ClassFileStructure structure, ByteBuffer data) {
        structure.methods = new MethodInfo[structure.methodsCount];
        for (int i = 0; i < structure.methodsCount; i++) {
            structure.methods[i] = new MethodInfo().parse(data);
        }
    }

2.15 attributesCount

    private void attributesCount(ClassFileStructure structure, ByteBuffer data) {
        structure.attributesCount = readUnsignedShort(data);
    }

2.16 AttributeInfo[]

    private void attributes(ClassFileStructure structure, ByteBuffer data) {
        structure.attributes = new AttributeInfo[structure.attributesCount];
        for (int i = 0; i < structure.attributesCount; i++) {
            structure.attributes[i] = new AttributeInfo().parse(data);
        }
    }