各种同步方法性能比较(synchronized,ReentrantLock,Atomic)

5.0的多线程任务包对于同步的性能方面有了很大的改进，在原有synchronized关键字的基础上，又增加了ReentrantLock，以及各种Atomic类。了解其性能的优劣程度，有助与我们在特定的情形下做出正确的选择。 

总体的结论先摆出来：  

synchronized： 
在资源竞争不是很激烈的情况下，偶尔会有同步的情形下，synchronized是很合适的。原因在于，编译程序通常会尽可能的进行优化synchronize，另外可读性非常好，不管用没用过5.0多线程包的程序员都能理解。 

ReentrantLock: 
ReentrantLock提供了多样化的同步，比如有时间限制的同步，可以被Interrupt的同步（synchronized的同步是不能Interrupt的）等。在资源竞争不激烈的情形下，性能稍微比synchronized差点点。但是当同步非常激烈的时候，synchronized的性能一下子能下降好几十倍。而ReentrantLock确还能维持常态。 

Atomic: 
和上面的类似，不激烈情况下，性能比synchronized略逊，而激烈的时候，也能维持常态。激烈的时候，Atomic的性能会优于ReentrantLock一倍左右。但是其有一个缺点，就是只能同步一个值，一段代码中只能出现一个Atomic的变量，多于一个同步无效。因为他不能在多个Atomic之间同步。 


所以，我们写同步的时候，优先考虑synchronized，如果有特殊需要，再进一步优化。ReentrantLock和Atomic如果用的不好，不仅不能提高性能，还可能带来灾难。 

先贴测试结果：再贴代码（Atomic测试代码不准确，一个同步中只能有1个Actomic，这里用了2个，但是这里的测试只看速度） 
==========================
round:100000 thread:5
Sync  = 96515266
Lock  = 25434694
Atom = 22142464
==========================
round:200000 thread:10
Sync  = 363174894
Lock  = 92003568
Atom = 60405932
==========================
round:300000 thread:15
Sync  = 954456020
Lock  = 184936307
Atom = 141182490
==========================
round:400000 thread:20
Sync  = 1439020073
Lock  = 372073298
Atom = 328126317
==========================
round:500000 thread:25
Sync  = 2807426174
Lock  = 550143645
Atom = 427540885

package zmx.atomic.test;import java.util.Random;import java.util.concurrent.BrokenBarrierException;import java.util.concurrent.CyclicBarrier;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.atomic.AtomicInteger;import java.util.concurrent.atomic.AtomicLong;import java.util.concurrent.locks.ReentrantLock;public class TestSyncMethods {public static void test(int round, int threadNum,CyclicBarrier cyclicBarrier) {new SyncTest("Sync", round, threadNum, cyclicBarrier).testTime();new LockTest("Lock", round, threadNum, cyclicBarrier).testTime();new AtomicTest("Atom", round, threadNum, cyclicBarrier).testTime();}public static void main(String args[]) {for (int i = 0; i < 5; i++) {int round = 100000 * (i + 1);int threadNum = 5 * (i + 1);CyclicBarrier cb = new CyclicBarrier(threadNum * 2 + 1);System.out.println("==========================");System.out.println("round:" + round + " thread:" + threadNum);test(round, threadNum, cb);}}}class SyncTest extends TestTemplate {public SyncTest(String _id, int _round, int _threadNum, CyclicBarrier _cb) {super(_id, _round, _threadNum, _cb);}/** * synchronized关键字不在方法签名里面，所以不涉及重载问题 */@Overridesynchronized long getValue() {return super.countValue;}@Overridesynchronized void sumValue() {super.countValue += preInit[index++ % round];}}class LockTest extends TestTemplate {ReentrantLock lock = new ReentrantLock();public LockTest(String _id, int _round, int _threadNum, CyclicBarrier _cb) {super(_id, _round, _threadNum, _cb);}/** * synchronized关键字不在方法签名里面，所以不涉及重载问题 */    @Overridelong getValue() {try {lock.lock();return super.countValue;} finally {lock.unlock();}}@Overridevoid sumValue() {try {lock.lock();super.countValue += preInit[index++ % round];} finally {lock.unlock();}}} class AtomicTest extends TestTemplate {public AtomicTest(String _id, int _round, int _threadNum, CyclicBarrier _cb) {super(_id, _round, _threadNum, _cb);}/** * synchronized关键字不在方法签名里面，所以不涉及重载问题 */@Overridelong getValue() {return super.countValueAtmoic.get();}@Overridevoid sumValue() {super.countValueAtmoic.addAndGet(super.preInit[indexAtomic.get()% round]);}}abstract class TestTemplate {private String id;protected int round;private int threadNum;protected long countValue;protected AtomicLong countValueAtmoic = new AtomicLong(0);protected int[] preInit;protected int index;protected AtomicInteger indexAtomic = new AtomicInteger(0);Random r = new Random(47);// 任务栅栏，同批任务，先到达wait的任务挂起，一直等到全部任务到达制定的wait地点后，才能全部唤醒，继续执行private CyclicBarrier cb;public TestTemplate(String _id, int _round, int _threadNum,CyclicBarrier _cb) {this.id = _id;this.round = _round;this.threadNum = _threadNum;cb = _cb;preInit = new int[round];for (int i = 0; i < preInit.length; i++) {preInit[i] = r.nextInt(100);}}abstract void sumValue();/* * 对long的操作是非原子的，原子操作只针对32位 long是64位，底层操作的时候分2个32位读写，因此不是线程安全 */abstract long getValue();public void testTime() {ExecutorService se = Executors.newCachedThreadPool();long start = System.nanoTime();// 同时开启2*ThreadNum个数的读写线程for (int i = 0; i < threadNum; i++) {se.execute(new Runnable() {public void run() {for (int i = 0; i < round; i++) {sumValue();}// 每个线程执行完同步方法后就等待try {cb.await();} catch (InterruptedException e) {e.printStackTrace();} catch (BrokenBarrierException e) {e.printStackTrace();}}});se.execute(new Runnable() {public void run() {getValue();try {// 每个线程执行完同步方法后就等待cb.await();} catch (InterruptedException e) {e.printStackTrace();} catch (BrokenBarrierException e) {e.printStackTrace();}}});}try {// 当前统计线程也wait,所以CyclicBarrier的初始值是threadNum*2+1cb.await();} catch (InterruptedException e) {e.printStackTrace();} catch (BrokenBarrierException e) {e.printStackTrace();}// 所有线程执行完成之后，才会跑到这一步long duration = System.nanoTime() - start;System.out.println(id + " = " + duration);}}

另外看一下synchronized对性能的影响有多大:

class SyncTest {    private static Object lock = newObject();      public static void main(String[] args) {        for(int k = 0; k < 10; k++) {            long start = System.currentTimeMillis();            for(int j = 0; j < 10000000; j++) {                synchronized(lock) {                    inti = 1;                }            }            System.out.println(System.currentTimeMillis() - start);             start = System.currentTimeMillis();            for(int j = 0; j < 10000000; j++) {                int i = 1;            }            System.out.println(System.currentTimeMillis() - start);        }     }}

运行结果:301 
4 
307 
5 
297 
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290 
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295 
3 
296 
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302 
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296 
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299 
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294 
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