實(shí)際工作中，經(jīng)常會遇到多線程并發(fā)時的類似搶購的功能，本篇描述一個簡單的redis分布式鎖實(shí)現(xiàn)的多線程搶票功能。
直接上代碼。首先按照慣例，給出一個錯誤的示范：
我們可以看看，當(dāng)20個線程一起來搶10張票的時候，會發(fā)生什么事。
package com.tiger.utils;
public class TestMutilThread {
// 總票量
public static int count = 10;
public static void main(String[] args) {
statrtMulti();
}
public static void statrtMulti() {
for (int i = 1; i = 20; i++) {
TicketRunnable tickrunner = new TicketRunnable();
Thread thread = new Thread(tickrunner, "Thread No: " + i);
thread.start();
}
}
public static class TicketRunnable implements Runnable {
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + " start "
+ count);
// TODO Auto-generated method stub
// logger.info(Thread.currentThread().getName()
// + " really start" + count);
if (count = 0) {
System.out.println(Thread.currentThread().getName()
+ " ticket sold out ! No tickets remained!" + count);
return;
} else {
count = count - 1;
System.out.println(Thread.currentThread().getName()
+ " bought a ticket,now remaining :" + (count));
}
}
}
}
測試結(jié)果，從結(jié)果可以看到，票數(shù)在不同的線程中已經(jīng)出現(xiàn)混亂。
Thread No: 2 start 10
Thread No: 6 start 10
Thread No: 4 start 10
Thread No: 5 start 10
Thread No: 3 start 10
Thread No: 9 start 6
Thread No: 1 start 10
Thread No: 1 bought a ticket,now remaining :3
Thread No: 9 bought a ticket,now remaining :4
Thread No: 3 bought a ticket,now remaining :5
Thread No: 12 start 3
Thread No: 5 bought a ticket,now remaining :6
Thread No: 4 bought a ticket,now remaining :7
Thread No: 8 start 7
Thread No: 7 start 8
Thread No: 12 bought a ticket,now remaining :1
Thread No: 14 start 0
Thread No: 6 bought a ticket,now remaining :8
Thread No: 16 start 0
Thread No: 2 bought a ticket,now remaining :9
Thread No: 16 ticket sold out ! No tickets remained!0
Thread No: 14 ticket sold out ! No tickets remained!0
Thread No: 18 start 0
Thread No: 18 ticket sold out ! No tickets remained!0
Thread No: 7 bought a ticket,now remaining :0
Thread No: 15 start 0
Thread No: 8 bought a ticket,now remaining :1
Thread No: 13 start 2
Thread No: 19 start 0
Thread No: 11 start 3
Thread No: 11 ticket sold out ! No tickets remained!0
Thread No: 10 start 3
Thread No: 10 ticket sold out ! No tickets remained!0
Thread No: 19 ticket sold out ! No tickets remained!0
Thread No: 13 ticket sold out ! No tickets remained!0
Thread No: 20 start 0
Thread No: 20 ticket sold out ! No tickets remained!0
Thread No: 15 ticket sold out ! No tickets remained!0
Thread No: 17 start 0
Thread No: 17 ticket sold out ! No tickets remained!0
為了解決多線程時出現(xiàn)的混亂問題，這里給出真正的測試類!!!
真正的測試類，這里啟動20個線程，來搶10張票。
RedisTemplate 是用來實(shí)現(xiàn)redis操作的，由spring進(jìn)行集成。這里是使用到了RedisTemplate，所以我以構(gòu)造器的形式在外部將RedisTemplate傳入到測試類中。
MultiTestLock 是用來實(shí)現(xiàn)加鎖的工具類。
總票數(shù)使用volatile關(guān)鍵字，實(shí)現(xiàn)多線程時變量在系統(tǒng)內(nèi)存中的可見性，這點(diǎn)可以去了解下volatile關(guān)鍵字的作用。
TicketRunnable用于模擬搶票功能。
其中由于lock與unlock之間存在if判斷，為保證線程安全，這里使用synchronized來保證。
測試類：
package com.tiger.utils;
import java.io.Serializable;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.data.redis.core.RedisTemplate;
public class MultiConsumer {
Logger logger=LoggerFactory.getLogger(MultiTestLock.class);
private RedisTemplateSerializable, Serializable> redisTemplate;
public MultiTestLock lock;
//總票量
public volatile static int count = 10;
public void statrtMulti() {
lock = new MultiTestLock(redisTemplate);
for (int i = 1; i = 20; i++) {
TicketRunnable tickrunner = new TicketRunnable();
Thread thread = new Thread(tickrunner, "Thread No: " + i);
thread.start();
}
}
public class TicketRunnable implements Runnable {
@Override
public void run() {
logger.info(Thread.currentThread().getName() + " start "
+ count);
// TODO Auto-generated method stub
if (count > 0) {
// logger.info(Thread.currentThread().getName()
// + " really start" + count);
lock.lock();
synchronized (this) {
if(count=0){
logger.info(Thread.currentThread().getName()
+ " ticket sold out ! No tickets remained!" + count);
lock.unlock();
return;
}else{
count=count-1;
logger.info(Thread.currentThread().getName()
+ " bought a ticket,now remaining :" + (count));
}
}
lock.unlock();
}else{
logger.info(Thread.currentThread().getName()
+ " ticket sold out !" + count);
}
}
}
public RedisTemplateSerializable, Serializable> getRedisTemplate() {
return redisTemplate;
}
public void setRedisTemplate(
RedisTemplateSerializable, Serializable> redisTemplate) {
this.redisTemplate = redisTemplate;
}
public MultiConsumer(RedisTemplateSerializable, Serializable> redisTemplate) {
super();
this.redisTemplate = redisTemplate;
}
}
Lock工具類：
我們知道為保證線程安全，程序中執(zhí)行的操作必須時原子的。redis后續(xù)的版本中可以使用set key同時設(shè)置expire超時時間。
想起上次去 電信翼支付 面試時，面試官問過一個問題：分布式鎖如何防止死鎖，問題關(guān)鍵在于我們在分布式中進(jìn)行加鎖操作時成功了，但是后續(xù)業(yè)務(wù)操作完畢執(zhí)行解鎖時出現(xiàn)失敗。導(dǎo)致分布式鎖無法釋放。出現(xiàn)死鎖，后續(xù)的加鎖無法正常進(jìn)行。所以這里設(shè)置expire超時時間的目的就是防止出現(xiàn)解鎖失敗的情況，這樣，即使解鎖失敗了，分布式鎖依然會在超時時間過了之后自動釋放。
具體在代碼中也有注釋，也可以作為參考。
package com.tiger.utils;
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Random;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import javax.sound.midi.MidiDevice.Info;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.dao.DataAccessException;
import org.springframework.data.redis.core.RedisOperations;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.data.redis.core.SessionCallback;
import org.springframework.data.redis.core.script.RedisScript;
public class MultiTestLock implements Lock {
Logger logger=LoggerFactory.getLogger(MultiTestLock.class);
private RedisTemplateSerializable, Serializable> redisTemplate;
public MultiTestLock(RedisTemplateSerializable, Serializable> redisTemplate) {
super();
this.redisTemplate = redisTemplate;
}
@Override
public void lock() {
//這里使用while循環(huán)強(qiáng)制線程進(jìn)來之后先進(jìn)行搶鎖操作。只有搶到鎖才能進(jìn)行后續(xù)操作
while(true){
if(tryLock()){
try {
//這里讓線程睡500毫秒的目的是為了模擬業(yè)務(wù)耗時，確保業(yè)務(wù)結(jié)束時之前設(shè)置的值正好打到超時時間，
//實(shí)際生產(chǎn)中可能有偏差，這里需要經(jīng)驗(yàn)
Thread.sleep(500l);
// logger.info(Thread.currentThread().getName()+" time to awake");
return;
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}else{
try {
//這里設(shè)置一個隨機(jī)毫秒的sleep目的時降低while循環(huán)的頻率
Thread.sleep(new Random().nextInt(200)+100);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
@Override
public boolean tryLock() {
//這里也可以選用transactionSupport支持事務(wù)操作
SessionCallbackObject> sessionCallback=new SessionCallbackObject>() {
@Override
public Object execute(RedisOperations operations)
throws DataAccessException {
operations.multi();
operations.opsForValue().setIfAbsent("secret", "answer");
//設(shè)置超時時間要根據(jù)業(yè)務(wù)實(shí)際的可能處理時間來，是一個經(jīng)驗(yàn)值
operations.expire("secret", 500l, TimeUnit.MILLISECONDS);
Object object=operations.exec();
return object;
}
};
//執(zhí)行兩部操作，這里會拿到一個數(shù)組值 [true,true],分別對應(yīng)上述兩部操作的結(jié)果，如果中途出現(xiàn)第一次為false則表明第一步set值出錯
ListBoolean> result=(List) redisTemplate.execute(sessionCallback);
// logger.info(Thread.currentThread().getName()+" try lock "+ result);
if(true==result.get(0)||"true".equals(result.get(0)+"")){
logger.info(Thread.currentThread().getName()+" try lock success");
return true;
}else{
return false;
}
}
@Override
public boolean tryLock(long arg0, TimeUnit arg1)
throws InterruptedException {
// TODO Auto-generated method stub
return false;
}
@Override
public void unlock() {
//unlock操作直接刪除鎖，如果執(zhí)行完還沒有達(dá)到超時時間則直接刪除，讓后續(xù)的線程進(jìn)行繼續(xù)操作。起到補(bǔ)刀的作用，確保鎖已經(jīng)超時或被刪除
SessionCallbackObject> sessionCallback=new SessionCallbackObject>() {
@Override
public Object execute(RedisOperations operations)
throws DataAccessException {
operations.multi();
operations.delete("secret");
Object object=operations.exec();
return object;
}
};
Object result=redisTemplate.execute(sessionCallback);
}
@Override
public void lockInterruptibly() throws InterruptedException {
// TODO Auto-generated method stub
}
@Override
public Condition newCondition() {
// TODO Auto-generated method stub
return null;
}
public RedisTemplateSerializable, Serializable> getRedisTemplate() {
return redisTemplate;
}
public void setRedisTemplate(
RedisTemplateSerializable, Serializable> redisTemplate) {
this.redisTemplate = redisTemplate;
}
}
執(zhí)行結(jié)果
可以看到，票數(shù)穩(wěn)步減少，后續(xù)沒有搶到鎖的線程余票為0，無票可搶。
tips:
這其中也出現(xiàn)了一個問題，redis進(jìn)行多部封裝操作時，系統(tǒng)報錯：ERR EXEC without MULTI
后經(jīng)過查閱發(fā)現(xiàn)問題出在：
在spring中，多次執(zhí)行MULTI命令不會報錯，因?yàn)榈谝淮螆?zhí)行時，會將其內(nèi)部的一個isInMulti變量設(shè)為true，后續(xù)每次執(zhí)行命令是都會檢查這個變量，如果為true，則不執(zhí)行命令。
而多次執(zhí)行EXEC命令則會報開頭說的"ERR EXEC without MULTI"錯誤。
以上為個人經(jīng)驗(yàn)，希望能給大家一個參考，也希望大家多多支持腳本之家。如有錯誤或未考慮完全的地方，望不吝賜教。
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