Unsafe源码深究

 1.Unsafe思维导图

由于 Unsafe类被final修饰符修饰，是一个不可以被继承的类；该类的属性有三种：偏移量，索引标度以及地址大小

2.源码分析

Unsafe所在包rt.jar sun.misc;

2.1 类属性大致分三种：偏移量，索引标度，地址大小。

public final class Unsafe {
    private static final Unsafe theUnsafe;
    public static final int INVALID_FIELD_OFFSET = -1;
    public static final int ARRAY_BOOLEAN_BASE_OFFSET;
    public static final int ARRAY_BYTE_BASE_OFFSET;
    public static final int ARRAY_SHORT_BASE_OFFSET;
    public static final int ARRAY_CHAR_BASE_OFFSET;
    public static final int ARRAY_INT_BASE_OFFSET;
    public static final int ARRAY_LONG_BASE_OFFSET;
    public static final int ARRAY_FLOAT_BASE_OFFSET;
    public static final int ARRAY_DOUBLE_BASE_OFFSET;
    public static final int ARRAY_OBJECT_BASE_OFFSET;
    public static final int ARRAY_BOOLEAN_INDEX_SCALE;
    public static final int ARRAY_BYTE_INDEX_SCALE;
    public static final int ARRAY_SHORT_INDEX_SCALE;
    public static final int ARRAY_CHAR_INDEX_SCALE;
    public static final int ARRAY_INT_INDEX_SCALE;
    public static final int ARRAY_LONG_INDEX_SCALE;
    public static final int ARRAY_FLOAT_INDEX_SCALE;
    public static final int ARRAY_DOUBLE_INDEX_SCALE;
    public static final int ARRAY_OBJECT_INDEX_SCALE;
    public static final int ADDRESS_SIZE;

2.2 初始化操作

    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class var0 = Reflection.getCallerClass();
        if (!VM.isSystemDomainLoader(var0.getClassLoader())) {
            throw new SecurityException("Unsafe");
        } else {
            return theUnsafe;
        }
    }

    // 可以看到getCallerClass是Reflection类中的本地方法
    // 这个方法的意思返回调用者 （class对象）,也就是说是谁调用了我这个类的方法
    @CallerSensitive
    public static native Class getCallerClass();
    // 可以看到isSystemDomainLoader是VM类中静态方法
    public static boolean isSystemDomainLoader(ClassLoader var0) {
        return var0 == null;
    }

• Reflection.getCallerClass()方法调用所在的方法必须用@CallerSensitive进行注解，通过此方法获取class时会跳过链路上所有的有@CallerSensitive注解的方法的类，直到遇到第一个未使用该注解的类，避免了用Reflection.getCallerClass(int n) 这个过时方法来自己做判断。
• Reflection.getCallerClass()此方法的调用者必须有权限，需要什么样的权限呢？
  • 由bootstrap class loader加载的类可以调用
  • 由extension class loader加载的类可以调用
• 都知道用户路径的类加载都是由 application class loader进行加载的，换句话说就是用户自定义的一些类中无法调用此方法

2.3 类方法

2.3.1 CAS相关操作

    public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);

    public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);

    public final native boolean compareAndSwapLong(Object var1, long var2, long var4, long var6);

2.3.2 直接内存操作

    // 分配内存
    public native long allocateMemory(long var1);
    // 重新分配内存
    public native long reallocateMemory(long var1, long var3);
    // 内存初始化
    public native void setMemory(long var1, long var3, byte var5);
    // 内存复制
    public native void copyMemory(Object var1, long var2, Object var4, long var5, long     var7);
    // 清除内存
    public native void freeMemory(long var1);

2.3.3有序写入操作

    public native void putOrderedObject(Object var1, long var2, Object var4);

    public native void putOrderedInt(Object var1, long var2, int var4);

    public native void putOrderedLong(Object var1, long var2, long var4);

2.3.4 volatile读写操作

    public native Object getObjectVolatile(Object var1, long var2);

    public native void putObjectVolatile(Object var1, long var2, Object var4);

    public native int getIntVolatile(Object var1, long var2);

    public native void putIntVolatile(Object var1, long var2, int var4);

    public native boolean getBooleanVolatile(Object var1, long var2);

    public native void putBooleanVolatile(Object var1, long var2, boolean var4);

    public native byte getByteVolatile(Object var1, long var2);

    public native void putByteVolatile(Object var1, long var2, byte var4);

    public native short getShortVolatile(Object var1, long var2);

    public native void putShortVolatile(Object var1, long var2, short var4);

    public native char getCharVolatile(Object var1, long var2);

    public native void putCharVolatile(Object var1, long var2, char var4);

    public native long getLongVolatile(Object var1, long var2);

    public native void putLongVolatile(Object var1, long var2, long var4);

    public native float getFloatVolatile(Object var1, long var2);

    public native void putFloatVolatile(Object var1, long var2, float var4);

    public native double getDoubleVolatile(Object var1, long var2);

    public native void putDoubleVolatile(Object var1, long var2, double var4);

2.3.5 普通读写操作

    public native int getInt(Object var1, long var2);

    public native void putInt(Object var1, long var2, int var4);

    public native Object getObject(Object var1, long var2);

    public native void putObject(Object var1, long var2, Object var4);

    public native boolean getBoolean(Object var1, long var2);

    public native void putBoolean(Object var1, long var2, boolean var4);

    public native byte getByte(Object var1, long var2);

    public native void putByte(Object var1, long var2, byte var4);

    public native short getShort(Object var1, long var2);

    public native void putShort(Object var1, long var2, short var4);

    public native char getChar(Object var1, long var2);

    public native void putChar(Object var1, long var2, char var4);

    public native long getLong(Object var1, long var2);

    public native void putLong(Object var1, long var2, long var4);

    public native float getFloat(Object var1, long var2);

    public native void putFloat(Object var1, long var2, float var4);

    public native double getDouble(Object var1, long var2);

    public native void putDouble(Object var1, long var2, double var4);

2.3.6 线程相关操作

    public native void unpark(Object var1);

    public native void park(boolean var1, long var2);

    /** @deprecated */
    @Deprecated
    public native void monitorEnter(Object var1);

    /** @deprecated */
    @Deprecated
    public native void monitorExit(Object var1);