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測試Java的synchronize和ReentrantLock在單線程下的效率
ReentrantLock 在多線程情況下要遠勝synchronize，這點沒有疑問。
最近要寫個程序，有個變量是有多數情況下是一個線程讀寫，有少數情況下是多個線程並發讀寫。所以要測試下ReentrantLock 在單線程下和synchronize的效率對比。
在測試的過程中發現一個有意思的現象。
測試代碼見後麵。
測試代碼1結果：
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
則開始看到這個測試結果，覺得相當奇怪，ReentrantLock居然比synchronize要慢這麼多。
但是後來再仔細研究測試結果，發現原來是jvm把測試代碼優化掉了！觀察noLockTime的時間，差不多是0！
於是加上total的統計代碼，得到以下的測試結果：
加上total統計的測試代碼結果：
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
這個結果算是正常的結果了。
總結：
- 以前一直聽說JVM可以在運行時優化代碼，果然是相當的牛B的功能，直接在運行時把無用的代碼全優化掉了。
- 測試程序要注意防止jvm等的優化
- ReentrantLock和synchronize在單線程下效率基本一樣（從實現原理來看，在單線程情況下，都是一個比較指令即可）
- 在單線程下，ReentrantLock和synchronize的效率是非常高的，一次調用約10e-11秒。
測試代碼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統計的測試代碼：
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();
}
}
}
最後更新：2017-04-02 16:47:37