1. Socket timeout
Java socket有如下两种timeout:
- 建立连接timeout,暂时就叫 connect timeout;
- 读取数据timeout,暂时就叫so timeout。
1.1 建立连接connect timeout
当不设置该参数时,指客户端请求和服务端建立tcp连接时,会一直阻塞直到连接建立成功,或抛异常。当设置了connectTimeout, 客户端请求和服务端建立连接时,阻塞时间超过connectTimeout时,就会抛出异常java.net.ConnectException: Connection timed out: connect。
我们看如下精简后的代码,首先是服务端:
serverSocket = new ServerSocket(8080);
Socket socket = serverSocket.accept();
服务端开启ServerSocket监听8080端口,再看客户端:
socket = new Socket();
socket.connect(new InetSocketAddress("localhost", 8080));
System.out.println("Connected.");
打印“Connected.”,修改客户端代码中的主机名为一个不存在的主机:
socket = new Socket();
long t1 = 0;
try {
t1 = System.currentTimeMillis();
socket.connect(new InetSocketAddress("www.ss.ssss", 8080));
} catch (IOException e) {
long t2 = System.currentTimeMillis();
e.printStackTrace();
System.out.println("Connect failed, take time -> " + (t2 - t1) + "ms.");
}
抛出异常:java.net.ConnectException: Connection timed out: connect,并打印:Connect failed, take time -> 18532ms. 也就是当未设置connect timeout时,connect方法会阻塞直到底层异常抛出。经过测试socket有个默认的超时时间,大概在20秒左右(测试的值,不一定准确,待研究JVM源码)。下面我们来设置connect timeout,再看看效果:
socket = new Socket();
long t1 = 0;
try {
t1 = System.currentTimeMillis();
// 设置connect timeout 为2000毫秒
socket.connect(new InetSocketAddress("www.ss.ssss", 8080), 2000);
} catch (IOException e) {
long t2 = System.currentTimeMillis();
e.printStackTrace();
System.out.println("Connect failed, take time -> " + (t2 - t1) + "ms.");
}
抛出异常:java.net.SocketTimeoutException: connect timed out,并打印:Connect failed, take time -> 2014ms. 这里就是connect timeout发挥作用了。
1.2 读取数据so timeout
先看下jdk源码注释:
Enable/disable SO_TIMEOUT with the specified timeout, in milliseconds. With this option set to a non-zero timeout, a read() call on the InputStream associated with this Socket will block for only this amount of time. If the timeout expires, a java.net.SocketTimeoutException is raised, though the Socket is still valid. The option must be enabled prior to entering the blocking operation to have effect. The timeout must be > 0. A timeout of zero is interpreted as an infinite timeout.
这个参数通过socket.setSoTimeout(int timeout)方法设置,可以看出它的意思是,socket关联的InputStream的read()方法会阻塞,直到超过设置的so timeout,就会抛出SocketTimeoutException。当不设置这个参数时,默认值为无穷大,即InputStream的read方法会一直阻塞下去,除非连接断开。
下面通过代码来看下效果:
服务端代码:
serverSocket = new ServerSocket(8080);
Socket socket = serverSocket.accept();
服务端只接受socket但不发送任何数据给客户端。客户端代码:
socket = new Socket();
socket.connect(new InetSocketAddress("localhost", 8080));
System.out.println("Connected.");
in = socket.getInputStream();
System.out.println("reading...");
in.read();
System.out.println("read end");
客户端建立连接就开始读取InputStream。打印:
Connected.
reading...
并且一直阻塞在in.read(); 上。接下来我设置so timeout,代码如下:
long t1 = 0;
try {
socket = new Socket();
socket.connect(new InetSocketAddress("localhost", 8080));
// 设置so timeout 为2000毫秒
socket.setSoTimeout(2000);
System.out.println("Connected.");
in = socket.getInputStream();
System.out.println("reading...");
t1 = System.currentTimeMillis();
in.read();
} catch (IOException e) {
long t2 = System.currentTimeMillis();
System.out.println("read end, take -> " + (t2 - t1) + "ms");
e.printStackTrace();
} finally {
if (this.reader != null) {
try {
this.reader.close();
} catch (IOException e) {
}
}
}
抛出异常:java.net.SocketTimeoutException: Read timed out, 打印:read end, take -> 2000ms , 说明so timeout起作用了。
1.3 小结
我们可以通过设置connect timeout来控制连接建立的超时时间(不是绝对的,当设置的主机名不合法,比如我设置主机名为abc,会抛异常java.net.UnknownHostException: abc,但是此时connect timeout设置是不起作用的,测试得出的结论,仅供参考)。
通过设置so timeout可以控制流读取数据的超时时间。
2. 使用案例
2.1 MySQL jdbc timeout
查阅MySQL Connector/J 5.1 Developer Guide 中的jdbc配置参数,有
connectTimeout
Timeout for socket connect (in milliseconds), with 0 being no timeout. Only works on JDK-1.4 or newer. Defaults to '0'.
Default: 0
Since version: 3.0.1
socketTimeout
Timeout on network socket operations (0, the default means no timeout).
Default: 0
Since version: 3.0.1
这两个参数分别就是对应上面我们分析的connect timeout和so timeout。
参数的设置方法有两种,一种是通过url设置,
jdbc:mysql://[host1][:port1][,[host2][:port2]]...[/[database]] [?propertyName1=propertyValue1[&propertyName2=propertyValue2]...]
即在url后面通过?加参数,比如jdbc:mysql://192.168.1.1:3306/test?connectTimeout=2000&socketTime=2000
还有一种方式是:
Properties info = new Properties();
info.put("user", this.username);
info.put("password", this.password);
info.put("connectTimeout", "2000");
info.put("socketTime", "2000");
return DriverManager.getConnection(this.url, info);
2.2 Jedis timeout
Jedis是最流行的redis java客户端工具,redis.clients.jedis.Jedis对象的构造器中就有参数设置,
public Jedis(final String host, final int port, final int connectionTimeout, final int soTimeout) {
super(host, port, connectionTimeout, soTimeout);
}
// 用一个参数timeout同时设置connect timeout 和 so timeout
public Jedis(final String host, final int port, final int timeout) {
super(host, port, timeout);
}
Jedis中so timeout个人觉得是有比较重要意义的,首先jedis so timeout默认值为2000毫秒,jedis的操作流程是客户端发送命令给客户端执行,然后客户端就开始执行InputStream.read()读取响应,当某个命令比较耗时(比如数据非常多的情况下执行“keys *”),而导致客户端迟迟没有收到响应,就可能导致java.net.SocketTimeoutException: Read timed out异常抛出。一般是不建议客户端执行非常耗时的命令,但是也不排除有这种特殊逻辑,那这时候就有可能需要修改Jeids中这个so timeout的值。
3. 总结
了解了这两个timeout之后,可以更好的处理一些网络服务的客户端和服务端,同时对排查一些问题也很有帮助。一般的成熟的网络服务和客户端都应该有这两个参数的配置方法,当使用遇到类似问题可以从这个方向去考虑下。