测试Java的synchronize和ReentrantLock在单线程下的效率

ReentrantLock 在多线程情况下要远胜synchronize，这点没有疑问。

最近要写个程序，有个变量是有多数情况下是一个线程读写，有少数情况下是多个线程并发读写。所以要测试下ReentrantLock 在单线程下和synchronize的效率对比。

在测试的过程中发现一个有意思的现象。

测试代码见后面。

测试代码1结果：

noLockTime: 0:00:00.004
noLockTime: 0:00:00.006
noLockTime: 0:00:00.000
syncTime: 0:00:02.902
syncTime: 0:00:02.902
syncTime: 0:00:02.886
ReentrantTime: 0:00:05.012
ReentrantTime: 0:00:05.007
ReentrantTime: 0:00:04.270

则开始看到这个测试结果，觉得相当奇怪，ReentrantLock居然比synchronize要慢这么多。

但是后来再仔细研究测试结果，发现原来是jvm把测试代码优化掉了！观察noLockTime的时间，差不多是0！

于是加上total的统计代码，得到以下的测试结果：

加上total统计的测试代码结果：

noLockTime: 0:00:03.786
total:1078760960
noLockTime: 0:00:03.774
total:1078760960
noLockTime: 0:00:03.428
total:1078760960
syncTime: 0:00:05.553
total:1078760960
syncTime: 0:00:05.555
total:1078760960
syncTime: 0:00:04.826
total:1078760960
ReentrantTime: 0:00:05.451
total:1078760960
ReentrantTime: 0:00:05.424
total:1078760960
ReentrantTime: 0:00:04.493
total:1078760960

这个结果算是正常的结果了。

总结：

1. 以前一直听说JVM可以在运行时优化代码，果然是相当的牛B的功能，直接在运行时把无用的代码全优化掉了。
2. 测试程序要注意防止jvm等的优化
3. ReentrantLock和synchronize在单线程下效率基本一样（从实现原理来看，在单线程情况下，都是一个比较指令即可）
4. 在单线程下，ReentrantLock和synchronize的效率是非常高的，一次调用约10e-11秒。

测试代码1：

import java.util.concurrent.locks.ReentrantLock;

import org.apache.commons.lang3.time.StopWatch;

public class XXX {
	static ReentrantLock lock = new ReentrantLock(false);
	static int size = 100;
	static int count = 100000000;
	
	static StopWatch watch = new StopWatch();
	public static void main(String[] args) {
		noLockTime();
		noLockTime();
		noLockTime();
		
		syncTime();
		syncTime();
		syncTime();
		
		ReentrantTime();
		ReentrantTime();
		ReentrantTime();
	}
	
	static void noLockTime(){
//		int total = 0;
		watch.reset();
		watch.start();
		for(int i = 0; i < count; ++i){
			testNotLock();
//			total += testNotLock();
		}
		watch.stop();
		System.out.println("noLockTime: " + watch);
//		System.out.println("total:" + total);
	}
	
	static void syncTime(){
//		int total = 0;
		watch.reset();
		watch.start();
		for(int i = 0; i < count; ++i){
			testSync();
//			total += testSync();
		}
		watch.stop();
		System.out.println("syncTime: " + watch);
//		System.out.println("total:" + total);
	}
	
	static void ReentrantTime(){
//		int total = 0;
		watch.reset();
		watch.start();
		for(int i = 0; i < count; ++i){
			testReentrantLock();
//			total += testReentrantLock();
		}
		watch.stop();
		System.out.println("ReentrantTime: " + watch);
//		System.out.println("total:" + total);
	}
	
	static int testNotLock(){
		int sum = 0;
		for(int i = 0; i < size; ++i){
			sum += i;
		}
		return sum;
	}
	
	static synchronized int testSync(){
		int sum = 0;
		for(int i = 0; i < size; ++i){
			sum += i;
		}
		return sum;
	}
	
	static int testReentrantLock(){
		try{
			lock.lock();
			int sum = 0;
			for(int i = 0; i < size; ++i){
				sum += i;
			}
			return sum;
		}finally{
			lock.unlock();
		}
	}
	
}

加上total统计的测试代码：

import java.util.concurrent.locks.ReentrantLock;

import org.apache.commons.lang3.time.StopWatch;

public class XXX {
	static ReentrantLock lock = new ReentrantLock(false);
	static int size = 100;
	static int count = 100000000;
	
	static StopWatch watch = new StopWatch();
	public static void main(String[] args) {
		noLockTime();
		noLockTime();
		noLockTime();
		
		syncTime();
		syncTime();
		syncTime();
		
		ReentrantTime();
		ReentrantTime();
		ReentrantTime();
	}
	
	static void noLockTime(){
		int total = 0;
		watch.reset();
		watch.start();
		for(int i = 0; i < count; ++i){
//			testNotLock();
			total += testNotLock();
		}
		watch.stop();
		System.out.println("noLockTime: " + watch);
		System.out.println("total:" + total);
	}
	
	static void syncTime(){
		int total = 0;
		watch.reset();
		watch.start();
		for(int i = 0; i < count; ++i){
//			testSync();
			total += testSync();
		}
		watch.stop();
		System.out.println("syncTime: " + watch);
		System.out.println("total:" + total);
	}
	
	static void ReentrantTime(){
		int total = 0;
		watch.reset();
		watch.start();
		for(int i = 0; i < count; ++i){
//			testReentrantLock();
			total += testReentrantLock();
		}
		watch.stop();
		System.out.println("ReentrantTime: " + watch);
		System.out.println("total:" + total);
	}
	
	static int testNotLock(){
		int sum = 0;
		for(int i = 0; i < size; ++i){
			sum += i;
		}
		return sum;
	}
	
	static synchronized int testSync(){
		int sum = 0;
		for(int i = 0; i < size; ++i){
			sum += i;
		}
		return sum;
	}
	
	static int testReentrantLock(){
		try{
			lock.lock();
			int sum = 0;
			for(int i = 0; i < size; ++i){
				sum += i;
			}
			return sum;
		}finally{
			lock.unlock();
		}
	}
	
}