Unsafe类

一、Unsafe类仅能被BootstrapClassLoader加载的类实例化，用户建的类默认都是ApplicationClassLoader加载的，实例化Unsafe时会报错（可直接管理内存，不对用户开放使用）。可以用反射实例化（方式一）。补充：加启动参数指定当前类由BootstrapClassLoader加载（方式二）

    private Unsafe() {//构造方法私有化，与下面static构造单例
    }

    @CallerSensitive
    public static Unsafe getUnsafe() {
        Class var0 = Reflection.getCallerClass();
        if (!VM.isSystemDomainLoader(var0.getClassLoader())) {//判断是否BootstrapClassLoader加载的类执行实例化的
            throw new SecurityException("Unsafe");
        } else {
            return theUnsafe;
        }
    }

    static {
        registerNatives();
        Reflection.registerMethodsToFilter(Unsafe.class, new String[]{"getUnsafe"});
        theUnsafe = new Unsafe();
        ARRAY_BOOLEAN_BASE_OFFSET = theUnsafe.arrayBaseOffset(boolean[].class);
        ARRAY_BYTE_BASE_OFFSET = theUnsafe.arrayBaseOffset(byte[].class);
        ARRAY_SHORT_BASE_OFFSET = theUnsafe.arrayBaseOffset(short[].class);
        ARRAY_CHAR_BASE_OFFSET = theUnsafe.arrayBaseOffset(char[].class);
        ARRAY_INT_BASE_OFFSET = theUnsafe.arrayBaseOffset(int[].class);
        ARRAY_LONG_BASE_OFFSET = theUnsafe.arrayBaseOffset(long[].class);
        ARRAY_FLOAT_BASE_OFFSET = theUnsafe.arrayBaseOffset(float[].class);
        ARRAY_DOUBLE_BASE_OFFSET = theUnsafe.arrayBaseOffset(double[].class);
        ARRAY_OBJECT_BASE_OFFSET = theUnsafe.arrayBaseOffset(Object[].class);
        ARRAY_BOOLEAN_INDEX_SCALE = theUnsafe.arrayIndexScale(boolean[].class);
        ARRAY_BYTE_INDEX_SCALE = theUnsafe.arrayIndexScale(byte[].class);
        ARRAY_SHORT_INDEX_SCALE = theUnsafe.arrayIndexScale(short[].class);
        ARRAY_CHAR_INDEX_SCALE = theUnsafe.arrayIndexScale(char[].class);
        ARRAY_INT_INDEX_SCALE = theUnsafe.arrayIndexScale(int[].class);
        ARRAY_LONG_INDEX_SCALE = theUnsafe.arrayIndexScale(long[].class);
        ARRAY_FLOAT_INDEX_SCALE = theUnsafe.arrayIndexScale(float[].class);
        ARRAY_DOUBLE_INDEX_SCALE = theUnsafe.arrayIndexScale(double[].class);
        ARRAY_OBJECT_INDEX_SCALE = theUnsafe.arrayIndexScale(Object[].class);
        ADDRESS_SIZE = theUnsafe.addressSize();
    }

二、反射实现实例化

public class UnsafeTest2 {
    static final Unsafe unsafe;

    static final long stateOffset;

    private volatile long state = 0;

    static {
        try {
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            unsafe = (Unsafe)field.get(null);//反射得到Unsafe实例
            stateOffset = unsafe.objectFieldOffset(UnsafeTest.class.getDeclaredField("state"));
        }catch (Exception e){
            System.out.println(e.getLocalizedMessage());
            throw new Error(e);
        }
    }

    public static void main(String[] args){
        UnsafeTest2 test = new UnsafeTest2();
        Boolean sucess = unsafe.compareAndSwapInt(test,stateOffset,0,1);
        System.out.println(sucess);
    }
}

三、重要方法

long objectFieldOffset(Field field):返回指定的变量在所属类中的内存偏移地址

int arrayBaseOffset(Class arrayClass):获取数组中第一个元素的地址。

int arrayIndexScale(Class arrayClass):获取数组中第一个元素占用的字节。

boolean compareAndSwapLong(Object obj,long offset,long expect,long update):比较对象obj中偏移量为offset的变量的值是否与expect相等，相等则使用update更新，然后返回true,否则返回false。

public native long getLongvolatile(Object obj,long offset):获取obj对象中偏移量为offset的变量对应的volatile语义的值。

void putLongvolatile(Object obj,long offset,long value):设置obj对象中offset偏移的类型为long的field的值的value，支持volatile语义（可见性，有序性）

void putOrderedLong(Object obj,long offset,long value):设置obj对象中offset偏移地址对应的long型field的值为value，无volatile语义（可见性，有序性）

void park(boolean isAbsolute,long time):阻塞当前线程，其中参数isAbsolute等于false且time等于0时表示一直阻塞

void unpark(Object thread):唤醒调用park后阻塞的线程。

long getAndSetLong(Object obj,long offset,long update):获取对象obj中偏移量为offset的变量volatile语义的当前值，并设置变量volatile语义的值为update。

long getAndAddLong(Object obj,long offset,long addValue):获取对象obj中偏移量为offset的变量volatile语义的当前值，并设置变量的值为原始值+addValue。

参考自《Java并发编程之美》