Unsafe是Java無鎖操作的基石，在無鎖并發(fā)類中都少不了它們的身影，比如ConcurrentHashMap, ConcurrentLinkedQueue, 都是由Unsafe類來實(shí)現(xiàn)的。相對(duì)于與Java中的鎖，它基本無開銷，會(huì)原地等待。本文主要介紹下Unsafe中的主要操作。
1 compareAndSwap
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/*** 比較obj的offset處內(nèi)存位置中的值和期望的值，如果相同則更新。此更新是不可中斷的。* * @param obj 需要更新的對(duì)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與field的當(dāng)前值相同，設(shè)置filed的值為這個(gè)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); |
這個(gè)就是著名的CAS操作了，分為三步來做
- 獲取obj對(duì)象中為offset的偏移值，這里假設(shè)為realVal
- 比較realVal和expect
- 如果相同，將該值更新為update，否則不更新
CAS家族還包括有，compareAndSwapObject(), compareAndSwapLong(), compareAndSwapInt()等等
用AtomicInteger中一個(gè)經(jīng)典的例子來說明：
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public final int getAndAdd(int delta) { }//unsafe.getAndAddIntpublic final int getAndAddInt(Object var1, long var2, int var4) { int var5; do { /**獲取原始值*/ var5 = this.getIntVolatile(var1, var2); /**確認(rèn)原始值沒有被其它線程修改時(shí)，再執(zhí)行更新var5+var4操作*/ } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4)); return var5;} |
2 putOrder
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/*** * 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); |
將obj對(duì)象的偏移量為offset的位置修改為value，因?yàn)镴ava中沒有內(nèi)存操作，而Unsafe的這個(gè)操作正好補(bǔ)充了內(nèi)存操作的不足。也可以用于數(shù)組操作，比如ConcurrentHashMap中就大量用到了該操作
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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]; // 往數(shù)組下標(biāo)為0的位置，寫入s0: ss[0]=s0 UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0] |
需要注意的是obj需要設(shè)置為Volatile,否則對(duì)于其它線程會(huì)不可見
3 putXxxVolatile
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/*** * 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); |
感覺和putOrderInt一樣，因?yàn)楸仨氃O(shè)置為Volatile，否則有什么用呢？
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