Unsafe是Java无锁操作的基石,在无锁并发类中都少不了它们的身影,比如ConcurrentHashMap, ConcurrentLinkedQueue, 都是由Unsafe类来实现的。相对于与Java中的锁,它基本无开销,会原地等待。本文主要介绍下Unsafe中的主要操作。
1 compareAndSwap
/*** 比较obj的offset处内存位置中的值和期望的值,如果相同则更新。此更新是不可中断的。* * @param obj 需要更新的对象* @param offset obj中整型field的偏移量* @param expect 希望field中存在的值* @param update 如果期望值expect与field的当前值相同,设置filed的值为这个新值* @return 如果field的值被更改返回true*/public native boolean compareAndSwapInt(Object obj, long offset, int expect, int update);
这个就是著名的CAS操作了,分为三步来做
CAS家族还包括有,compareAndSwapObject(), compareAndSwapLong(), compareAndSwapInt()等等
用AtomicInteger中一个经典的例子来说明:
public final int getAndAdd(int delta) { return unsafe.getAndAddInt(this, valueOffset, delta);}//unsafe.getAndAddIntpublic final int getAndAddInt(Object var1, long var2, int var4) { int var5; do { /**获取原始值*/ var5 = this.getIntVolatile(var1, var2); /**确认原始值没有被其它线程修改时,再执行更新var5+var4操作*/ } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4)); return var5;}
2 putOrder
/*** * Sets the value of the integer field at the specified offset in the * supplied object to the given value. This is an ordered or lazy * version of <code>putIntVolatile(Object,long,int)</code>, which * doesn't guarantee the immediate visibility of the change to other * threads. It is only really useful where the integer field is * <code>volatile</code>, and is thus expected to change unexpectedly. * * @param obj the object containing the field to modify. * @param offset the offset of the integer field within <code>obj</code>. * @param value the new value of the field. * @see #putIntVolatile(Object,long,int) */ public native void putOrderedInt(Object obj, long offset, int value);
将obj对象的偏移量为offset的位置修改为value,因为Java中没有内存操作,而Unsafe的这个操作正好补充了内存操作的不足。也可以用于数组操作,比如ConcurrentHashMap中就大量用到了该操作
Segment<K,V> s0 = new Segment<K,V>(loadFactor, (int)(cap * loadFactor), (HashEntry<K,V>[])new HashEntry[cap]); Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize]; // 往数组下标为0的位置,写入s0: ss[0]=s0 UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
需要注意的是obj需要设置为Volatile,否则对于其它线程会不可见
3 putXxxVolatile
/*** * Sets the value of the integer field at the specified offset in the * supplied object to the given value, with volatile store semantics. * * @param obj the object containing the field to modify. * @param offset the offset of the integer field within <code>obj</code>. * @param value the new value of the field. */ public native void putIntVolatile(Object obj, long offset, int value);
感觉和putOrderInt一样,因为必须设置为Volatile,否则有什么用呢?
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