Is JAX-RS Client Thread Safe

I am not sure but I think this is a implementation-specific decision.

I couldn't find in the JAX-RS 2.0 specification nor in the Javadoc anything granting that javax.ws.rs.client.Client is thread-safe. But in the Resteasy (an implementor of JAX-RS) documentation I found:

One default decision made by HttpClient and adopted by Resteasy is the use of org.apache.http.impl.conn.SingleClientConnManager, which manages a single socket at any given time and which supports the use case in which one or more invocations are made serially from a single thread. For multithreaded applications, SingleClientConnManager may be replaced by org.apache.http.impl.conn.tsccm.ThreadSafeClientConnManager:

ClientConnectionManager cm = new ThreadSafeClientConnManager();HttpClient httpClient = new DefaultHttpClient(cm);ApacheHttpClient4Engine engine = new ApacheHttpClient4Engine(httpClient);

Source: http://docs.jboss.org/resteasy/docs/3.0.9.Final/userguide/html/RESTEasy_Client_Framework.html#transport_layer

Based in these information I guess that the answer for your question is likely to be "no".

PLEASE BE AWARE: Although this is the accepted answer, this is implementation specific and was correct for the Jersey 1 Client. For that you absolutely should share a single instance. Creating a client per request is a huge performance overhead

The JavaDoc is mostly answering your question already- yes it's thread-safe and you can and should reuse it. There can be a performance issue from not reusing it, i.e. if you create a Client for every HTTP request you make your performance will suck really bad.