HttpClient 之 CLOSE_WAIT
Using HttpClient Properly to Avoid CLOSE_WAIT TCP Connections
Apache HttpClient
is usually used like this in basic mode:
HttpClient httpClient = new HttpClient();
HttpMethod method = new GetMethod(uri);
try {
int statusCode = httpClient.executeMethod(method);
byte[] responseBody = method.getResponseBody();
// ...
return stuff;
} finally {
method.releaseConnection();
}
But this is not enough.
The issue is this: releasing the connection makes it available again to the HttpClient
instance, but does not close it, because HTTP 1.1 is used and it can pipeline further requests to the same host:port in the same connection.
Even though the server may have decided to close its end of the connection, the connection is still open on our client side and it will stay that way until an attempt to read from it is made (at which point the client will detect that the other end is closed). TCP works like that, there is the notion of a half-closed connection because close()
actually just means I will not send any more data, but you can still receive data from a connection that you closed but that has not yet been closed on the other end.
What happens next is that when the HttpClient
instance goes out of scope, it becomes available to the GC, but it will not be garbage collected immediately. Until the GC collects it, the socket connection held internally will stay open and the socket will be stuck in the CLOSE_WAIT state.
To fix this, the simplest way is to add: method.setRequestHeader("Connection", "close");
before executing the method. This will instruct HttpClient
to close the connection by itself once the full response has been received.
Another way is to do it in the finally
block:
httpClient.getHttpConnectionManager().closeIdleConnections(0);
An even better way is to not use a new HttpClient
object each time, but to reuse one that has been initialized with a MultiThreadedHttpConnectionManager
sized appropriately.
Of course in this case the connection manager must be shut down properly when the application shuts down:
private MultiThreadedHttpConnectionManager connectionManager;
private HttpClient httpClient;
public void init() {
connectionManager = new MultiThreadedHttpConnectionManager()
// ... configure connectionManager ...
httpClient = new HttpClient(connectionManager);
}
public void shutdown() {
connectionManager.shutdown();
}
public String process(String uri) {
HttpMethod method = new GetMethod(uri);
try {
int statusCode = httpClient.executeMethod(method);
byte[] responseBody = method.getResponseBody();
// ...
return stuff;
} finally {
method.releaseConnection();
}
}