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<div class="title">Multithreading in Java </div> </div>
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<div class="textblock"><p>All Connection methods are thread safe, and Connection handles can be shared between threads. Applications typically open a single connection to each database, per process. Multi-threaded programs must wait for all other threads to exit before closing the Connection handle because that will implicitly close all other handles. Alternatively, a thread can set an event handler using Connection.open_session to be notified when a Connection close is in progress.</p>
<p>Session and Cursor methods are not thread safe and Session and Cursor handles cannot be accessed concurrently by multiple threads. Applications typically open one Session handle for each thread accessing a database, and then one or more Cursor handles within the session.</p>
<p>Session and Cursor methods may be accessed by different threads serially (for example, a pool of threads managed by the application with a set of shared session or cursor handles). There is no thread-local state in WiredTiger, and no built-in synchronization of session or cursor handles, either, so if multiple threads access a session or cursor handle, access must be serialized by the application.</p>
<p>AsyncOp methods are not thread-safe, and must be accessed by only a single thread at a time. AsyncOp methods may be accessed by different threads serially (and that is expected to happen when the asynchronous callback function runs).</p>
<h1><a class="anchor" id="threads_example_lang_java"></a>
Code samples</h1>
<p>The code below is taken from the complete example program ex_thread.java.</p>
<p>This is an example of a thread entry point. A new session is opened for the thread and used for all operations within that thread.</p>
<div class="fragment"><div class="line"><span class="keyword">class </span>ScanThread <span class="keyword">extends</span> Thread {</div><div class="line"> <span class="keyword">private</span> Connection conn;</div><div class="line"></div><div class="line"> <span class="keyword">public</span> ScanThread(Connection conn) {</div><div class="line"> this.conn = conn;</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="keyword">public</span> <span class="keywordtype">void</span> run()</div><div class="line"> {</div><div class="line"> <span class="keywordflow">try</span> {</div><div class="line"> <span class="keywordtype">int</span> ret;</div><div class="line"></div><div class="line"> Session session = conn.open_session(null);</div><div class="line"> Cursor cursor = session.open_cursor(<span class="stringliteral">"table:access"</span>, null, null);</div><div class="line"></div><div class="line"> <span class="comment">/* Show all records. */</span></div><div class="line"> <span class="keywordflow">while</span> ((ret = cursor.next()) == 0) {</div><div class="line"> String key = cursor.getKeyString();</div><div class="line"> String value = cursor.getValueString();</div><div class="line"> System.out.println(<span class="stringliteral">"Got record: "</span> + key + <span class="stringliteral">" : "</span> + value);</div><div class="line"> }</div><div class="line"> <span class="keywordflow">if</span> (ret != wiredtiger.WT_NOTFOUND)</div><div class="line"> System.err.println(<span class="stringliteral">"Cursor.next: "</span> +</div><div class="line"> wiredtiger.wiredtiger_strerror(ret));</div><div class="line"> cursor.<a class="code" href="struct_w_t___c_u_r_s_o_r.html#aeea071f192cab12245a50fbe71c3460b">close</a>();</div><div class="line"> session.close(null);</div><div class="line"> } <span class="keywordflow">catch</span> (WiredTigerException wte) {</div><div class="line"> System.err.println(<span class="stringliteral">"Exception "</span> + wte);</div><div class="line"> }</div><div class="line"> }</div><div class="line">}</div></div><!-- fragment --><p> Here is the main function that starts the threads. It opens a single connection, shared between the threads, and closes the connection after waiting for all of the threads to exit.</p>
<div class="fragment"><div class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keywordtype">void</span> main(String[] argv)</div><div class="line"> {</div><div class="line"> <span class="keywordflow">try</span> {</div><div class="line"> Thread[] threads = <span class="keyword">new</span> Thread[NUM_THREADS];</div><div class="line"> <span class="keywordtype">int</span> i, ret;</div><div class="line"> Connection conn;</div><div class="line"></div><div class="line"> <span class="comment">/*</span></div><div class="line"><span class="comment"> * Create a clean test directory for this run of the test program if the</span></div><div class="line"><span class="comment"> * environment variable isn't already set (as is done by make check).</span></div><div class="line"><span class="comment"> */</span></div><div class="line"> <span class="keywordflow">if</span> (System.getenv(<span class="stringliteral">"WIREDTIGER_HOME"</span>) == null) {</div><div class="line"> home = <span class="stringliteral">"WT_HOME"</span>;</div><div class="line"> <span class="keywordflow">try</span> {</div><div class="line"> Process proc = Runtime.getRuntime().exec(<span class="stringliteral">"/bin/rm -rf "</span> + home);</div><div class="line"> BufferedReader br = <span class="keyword">new</span> BufferedReader(</div><div class="line"> <span class="keyword">new</span> InputStreamReader(proc.getInputStream()));</div><div class="line"> <span class="keywordflow">while</span>(br.ready())</div><div class="line"> System.out.println(br.readLine());</div><div class="line"> br.close();</div><div class="line"> proc.waitFor();</div><div class="line"> <span class="keywordflow">if</span> (!(<span class="keyword">new</span> File(home)).mkdir())</div><div class="line"> System.err.println(<span class="stringliteral">"mkdir: failed"</span>);</div><div class="line"> } <span class="keywordflow">catch</span> (Exception ex) {</div><div class="line"> System.err.println(<span class="stringliteral">"Exception: "</span> + home + <span class="stringliteral">": "</span> + ex);</div><div class="line"> System.exit(1);</div><div class="line"> }</div><div class="line"> } <span class="keywordflow">else</span></div><div class="line"> home = null;</div><div class="line"></div><div class="line"> <span class="keywordflow">if</span> ((conn = wiredtiger.open(home, <span class="stringliteral">"create"</span>)) == null) {</div><div class="line"> System.err.println(<span class="stringliteral">"Error connecting to "</span> + home);</div><div class="line"> System.exit(1);</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="comment">/* Note: further error checking omitted for clarity. */</span></div><div class="line"></div><div class="line"> Session session = conn.open_session(null);</div><div class="line"> ret = session.create(<span class="stringliteral">"table:access"</span>, <span class="stringliteral">"key_format=S,value_format=S"</span>);</div><div class="line"> Cursor cursor = session.open_cursor(<span class="stringliteral">"table:access"</span>, null, <span class="stringliteral">"overwrite"</span>);</div><div class="line"> cursor.putKeyString(<span class="stringliteral">"key1"</span>);</div><div class="line"> cursor.putValueString(<span class="stringliteral">"value1"</span>);</div><div class="line"> ret = cursor.insert();</div><div class="line"> cursor.close();</div><div class="line"> ret = session.close(null);</div><div class="line"></div><div class="line"> <span class="keywordflow">for</span> (i = 0; i < NUM_THREADS; i++) {</div><div class="line"> threads[i] = <span class="keyword">new</span> ScanThread(conn);</div><div class="line"> threads[i].start();</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="keywordflow">for</span> (i = 0; i < NUM_THREADS; i++)</div><div class="line"> <span class="keywordflow">try</span> {</div><div class="line"> threads[i].join();</div><div class="line"> ret = -1;</div><div class="line"> }</div><div class="line"> <span class="keywordflow">catch</span> (InterruptedException ie) {</div><div class="line"> }</div><div class="line"></div><div class="line"> ret = conn.close(null);</div><div class="line"> System.exit(ret);</div><div class="line"> }</div><div class="line"> <span class="keywordflow">catch</span> (WiredTigerException wte) {</div><div class="line"> System.err.println(<span class="stringliteral">"Exception: "</span> + wte);</div><div class="line"> wte.printStackTrace();</div><div class="line"> System.exit(1);</div><div class="line"> }</div><div class="line"> }</div></div><!-- fragment --></div></div><!-- contents -->
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