Reusing JAX RS Client in multi-threaded environment (with resteasy)

Question

According to the documentation,

"Clients are heavy-weight objects that manage the client-side communication infrastructure. Initialization as well as disposal of a Client instance may be a rather expensive operation. It is therefore advised to construct only a small number of Client instances in the application. "

Ok, I'm trying to cache Client itself and WebTarget instances in a static variable, the someMethod() is invoked in multi-threaded environment:

private static Client client = ClientBuilder.newClient();
private static WebTarget webTarget = client.target("someBaseUrl");
...
public static String someMethod(String arg1, String arg2)
{
    WebTarget target = entrTarget.queryParam("arg1", arg1).queryParam("arg2", arg2);
    Response response = target.request().get();
    final String result = response.readEntity(String.class);
    response.close();
    return result;
}

But sometimes (not always) I'm get an exception:

Invalid use of BasicClientConnManager: connection still allocated. Make sure to release the connection before allocating another one.

How can Client/WebTarget be reused/cached correctly? Is it possible with JAX RS Client API? Or I have to use some framework-specific features (resteasy/jersey) Could you provide some example or documentation?

Answer 1

Your implementation is not thread-safe. When two threads access someMethod at the same time they are sharing the same Client and one will try to make a second request while the first one is not finished.

You have two choices:

• Synchronize the access to the Client and WebTarget manually.
• Let the container manage concurrency by annotating the enclosing type with @javax.ejb.Singleton which guarantees thread safety. (see chapter 4.8.5 of the EJB specification)

If someMethod in a container managed environment I would use the second approach.