网上都说jedis实例是非线程安全的，常常通过JedisPool连接池去管理实例，在多线程情况下让每个线程有自己独立的jedis实例，但都没有具体说明为啥jedis实例时非线程安全的，今天就让我们一探究竟吧！
1. jedis类图
jedis_note.png
2. 为什么jedis不是线程安全的？
由上述类图可知，Jedis类中有RedisInputStream和RedisOutputStream两个属性，而发送命令和获取返回值都是使用这两个成员变量，显然，这很容易引发多线程问题。测试代码如下：
public class BadConcurrentJedisTest {
private static final ExecutorService pool = Executors.newFixedThreadPool(20);
private static final Jedis jedis = new Jedis("192.168.58.99", 6379);
public static void main(String[] args) {
for(int i=0;i<20;i++){
pool.execute(new RedisSet());
}
}
static class RedisSet implements Runnable{
@Override
public void run() {
while(true){
jedis.set("hello", "world");
}
}
}
报错：
Exception in thread "pool-1-thread-4" redis.clients.jedis.exceptions.JedisConnectionException: java.net.SocketException: Socket Closed
at redis.clients.jedis.Connection.connect(Connection.java:164)
at redis.clients.jedis.BinaryClient.connect(BinaryClient.java:80)
at redis.clients.jedis.Connection.sendCommand(Connection.java:100)
at redis.clients.jedis.BinaryClient.set(BinaryClient.java:97)
at redis.clients.jedis.Client.set(Client.java:32)
at redis.clients.jedis.Jedis.set(Jedis.java:68)
at threadsafe.BadConcurrentJedisTest$RedisSet.run(BadConcurrentJedisTest.java:26)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.net.SocketException: Socket Closed
at java.net.AbstractPlainSocketImpl.setOption(AbstractPlainSocketImpl.java:212)
at java.net.Socket.setKeepAlive(Socket.java:1310)
at redis.clients.jedis.Connection.connect(Connection.java:149)
... 9 more
redis.clients.jedis.exceptions.JedisConnectionException: java.net.SocketException: Socket Closed
at redis.clients.jedis.Connection.connect(Connection.java:164)
at redis.clients.jedis.BinaryClient.connect(BinaryClient.java:80)
at redis.clients.jedis.Connection.sendCommand(Connection.java:100)
at redis.clients.jedis.BinaryClient.set(BinaryClient.java:97)
at redis.clients.jedis.Client.set(Client.java:32)
at redis.clients.jedis.Jedis.set(Jedis.java:68)
at threadsafe.BadConcurrentJedisTest$RedisSet.run(BadConcurrentJedisTest.java:26)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
.......
主要错误：
ava.net.SocketException: Socket closed
java.net.SocketException: Socket is not connected
2.1 共享socket引起的异常
为什么会出现这2个错误呢？ 我们可以很容易的通过堆栈信息定位到redis.clients.jedis.Connection的connect方法：
public void connect() {
if (!isConnected()) {
try {
socket = new Socket();
// ->@wjw_add
socket.setReuseAddress(true);
socket.setKeepAlive(true); // Will monitor the TCP connection is
// valid
socket.setTcpNoDelay(true); // Socket buffer Whetherclosed, to
// ensure timely delivery of data
socket.setSoLinger(true, 0); // Control calls close () method,
// the underlying socket is closed
// immediately
// <-@wjw_add
socket.connect(new InetSocketAddress(host, port), connectionTimeout);
socket.setSoTimeout(soTimeout);
if (ssl) {
if (null == sslSocketFactory) {
sslSocketFactory = (SSLSocketFactory)SSLSocketFactory.getDefault();
}
socket = (SSLSocket) sslSocketFactory.createSocket(socket, host, port, true);
if (null != sslParameters) {
((SSLSocket) socket).setSSLParameters(sslParameters);
}
if ((null != hostnameVerifier) &&
(!hostnameVerifier.verify(host, ((SSLSocket) socket).getSession()))) {
String message = String.format(
"The connection to '%s' failed ssl/tls hostname verification.", host);
throw new JedisConnectionException(message);
}
}
outputStream = new RedisOutputStream(socket.getOutputStream());
inputStream = new RedisInputStream(socket.getInputStream());
} catch (IOException ex) {
broken = true;
throw new JedisConnectionException(ex);
}
}
}
jedis在执行每一个命令之前都会先执行connect方法，socket是一个共享变量，在多线程的情况下可能存在：线程1执行到了
outputStream = new RedisOutputStream(socket.getOutputStream());
inputStream = new RedisInputStream(socket.getInputStream());
线程2执行到了：
socket = new Socket();
线程2
socket.connect(new InetSocketAddress(host, port), connectionTimeout);
因为线程2重新初始化了socket但是还没有执行connect，所以线程1执行socket.getOutputStream()或者socket.getInputStream()就会抛出java.net.SocketException: Socket is not connected。java.net.SocketException: Socket closed是因为socket异常导致共享变量socket关闭了引起的。
2.2 共享数据流引起的异常
上面是因为多个线程共享jedis引起的socket异常。除了socket连接引起的异常之外，还有共享数据流引起的异常。下面就看一下，因为共享jedis实例引起的共享数据流错误问题。
为了避免多线程连接的时候引起的错误，我们在初始化的时候就先执行一下connect操作：
public class BadConcurrentJedisTest1 {
private static final ExecutorService pool = Executors.newCachedThreadPool();
private static final Jedis jedis = new Jedis("192.168.58.99", 6379);
static{
jedis.connect();
}
public static void main(String[] args) {
for(int i=0;i<20;i++){
pool.execute(new RedisTest());
}
}
static class RedisTest implements Runnable{
@Override
public void run() {
while(true){
jedis.set("hello", "world");
}
}
}
}
报错：（每次报的错可能不完全一样）
Exception in thread "pool-1-thread-7" Exception in thread "pool-1-thread-1" redis.clients.jedis.exceptions.JedisDataException: ERR Protocol error: invalid multibulk length
at redis.clients.jedis.Protocol.processError(Protocol.java:123)
at redis.clients.jedis.Protocol.process(Protocol.java:157)
at redis.clients.jedis.Protocol.read(Protocol.java:211)
at redis.clients.jedis.Connection.readProtocolWithCheckingBroken(Connection.java:297)
at redis.clients.jedis.Connection.getStatusCodeReply(Connection.java:196)
at redis.clients.jedis.Jedis.set(Jedis.java:69)
at threadsafe.BadConcurrentJedisTest1$RedisTest.run(BadConcurrentJedisTest1.java:30)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Exception in thread "pool-1-thread-4" redis.clients.jedis.exceptions.JedisConnectionException: java.net.SocketException: Broken pipe (Write failed)
at redis.clients.jedis.Connection.flush(Connection.java:291)
at redis.clients.jedis.Connection.getStatusCodeReply(Connection.java:194)
at redis.clients.jedis.Jedis.set(Jedis.java:69)
at threadsafe.BadConcurrentJedisTest1$RedisTest.run(BadConcurrentJedisTest1.java:30)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.net.SocketException: Broken pipe (Write failed)
Protocol error: invalid multibulk lengt是因为多线程通过RedisInputStream和RedisOutputStream读写缓冲区的时候引起的问题造成的数据问题不满足RESP协议引起的。举个简单的例子，例如多个线程执行命令,线程1执行 set hello world命令。本来应该发送：
*3\r\n$3\r\nSET\r\n$5\r\nhello\r\n$5\r\nworld\r\n
但是线程执行写到
*3\r\n$3\r\nSET\r\n$5\r\nhello\r\n
然后被挂起了，线程2执行了写操作写入了' '，然后线程1继续执行，最后发送到redis服务器端的数据可能就是：
*3\r\n$3\r\nSET\r\n$5\r\nhello\r\n' '$5\r\nworld\r\n
至于java.net.SocketException: Connection reset或ReadTimeout错误，是因为redis服务器接受到错误的命令，执行了socket.close这样的操作，关闭了连接。服务器会返回复位标志"RST"，但是客户端还在继续执行读写数据操作。
3、jedis多线程操作
jedis本身不是多线程安全的，这并不是jedis的bug，而是jedis的设计与redis本身就是单线程相关，jedis实例抽象的是发送命令相关，一个jedis实例使用一个线程与使用100个线程去发送命令没有本质上的区别，所以没必要设置为线程安全的。但是如果需要用多线程方式访问redis服务器怎么做呢？那就使用多个jedis实例，每个线程对应一个jedis实例，而不是一个jedis实例多个线程共享。一个jedis关联一个Client，相当于一个客户端，Client继承了Connection，Connection维护了Socket连接，对于Socket这种昂贵的连接，一般都会做池化，jedis提供了JedisPool。
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import redis.clients.jedis.Jedis;
import redis.clients.jedis.JedisPool;
public class JedisPoolTest {
private static final ExecutorService pool = Executors.newCachedThreadPool();
private static final CountDownLatch latch = new CountDownLatch(20);
private static final JedisPool jPool = new JedisPool("192.168.58.99", 6379);
public static void main(String[] args) {
long start = System.currentTimeMillis();
for(int i=0;i<20;i++){
pool.execute(new RedisTest());
}
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(System.currentTimeMillis() - start);
pool.shutdownNow();
}
static class RedisTest implements Runnable{
@Override
public void run() {
Jedis jedis = jPool.getResource();
int i = 1000;
try{
while(i-->0){
jedis.set("hello", "world");
}
}finally{
jedis.close();
latch.countDown();
}
}
}
}
