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| <h1 id="exception-management">Exception Management<a class="headerlink" href="#exception-management" title="Permanent link">¶</a></h1> |
| <p>Managing Exceptions should not be a core part of your business logic. In fact with the Transaction Control |
| Service is doing its job you shouldn't need to worry about exceptions at all! A key design goal for the |
| Transaction Control Service is to avoid the need for try/catch/finally blocks as far as possible.</p> |
| <h2 id="throwing-exceptions-from-scoped-work">Throwing Exceptions from scoped work<a class="headerlink" href="#throwing-exceptions-from-scoped-work" title="Permanent link">¶</a></h2> |
| <p>Resources can be tempramental sometimes, and usually have defined exceptions that are thrown in certain |
| error cases. These can be generic, like <code>SQLException</code> or <code>JMSException</code>, |
| or specific like <code>EntityNotFoundException</code>.</p> |
| <p>In any event - exceptions indicate that a problem has occurred. By default an exception thrown from inside a |
| transactional scope will cause the transaction to roll back. This means that the code can safely ignore any |
| updates that were made. Furthermore, because a piece of scoped work is defined as a <code>Callable</code> |
| it is not necessary to catch or wrap an Exception raised in a scope.</p> |
| <div class="codehilite"><pre><span class="c1">// An SQLException may be raised by the query, </span> |
| <span class="c1">// but we don't need to catch it</span> |
| <span class="n">txControl</span><span class="p">.</span><span class="n">required</span><span class="p">(()</span> <span class="o">-></span> <span class="n">connection</span><span class="p">.</span><span class="n">createStatement</span><span class="p">()</span> |
| <span class="p">.</span><span class="n">executeQuery</span><span class="p">(</span><span class="s">"Insert into TEST_TABLE values ( 'Hello World!' )"</span><span class="p">));</span> |
| </pre></div> |
| |
| |
| <h2 id="catching-exceptions-thrown-from-scoped-work">Catching Exceptions thrown from scoped work<a class="headerlink" href="#catching-exceptions-thrown-from-scoped-work" title="Permanent link">¶</a></h2> |
| <p>In general Exceptions should not form part of your client API, so catching an Exception from a piece of scoped |
| work is rarely necessary. Usually exceptions generated by scoped work are eventually handled by catch all |
| collectors at the incoming request point (for example a servlet) and do not require special handling.</p> |
| <p>Sometimes, however, we have to work within an existing API that <em>does</em> use an Exception as a type of |
| return value. In that case it is important to know what happened to the Exception.</p> |
| <h3 id="the-scopedworkexception">The <code>ScopedWorkException</code><a class="headerlink" href="#the-scopedworkexception" title="Permanent link">¶</a></h3> |
| <p>Scoped work is free to throw checked or unchecked Exceptions, however these Exceptions cannot be directly |
| thrown on by the TransactionControl implementation. The primary reason for this is that directly rethrowing the |
| Exception would force users of the Transaction Control Service to either <em>always</em> declare |
| <code>throws Exception</code> on their methods or to add try/catch blocks around every call.</p> |
| <p>Exceptions generated as part of Scoped Work are therefore wrapped by the Transaction Control Service in a |
| <code>ScopedWorkException</code>. <code>ScopedWorkException</code> is an <em>unchecked exception</em> |
| and so can be ignored by your component if it does not require special handling (the typical case).</p> |
| <h3 id="unwrapping-the-scopedworkexception">Unwrapping the <code>ScopedWorkException</code><a class="headerlink" href="#unwrapping-the-scopedworkexception" title="Permanent link">¶</a></h3> |
| <p>As mentioned above, sometimes it is necessary for an API to throw a particular type of Exception as a return value.</p> |
| <p>This model can be supported by unwrapping the ScopedWorkException.</p> |
| <div class="codehilite"><pre><span class="k">try</span> <span class="p">{</span> |
| <span class="n">txControl</span><span class="p">.</span><span class="n">required</span><span class="p">(()</span> <span class="o">-></span> <span class="n">connection</span><span class="p">.</span><span class="n">createStatement</span><span class="p">()</span> |
| <span class="p">.</span><span class="n">executeQuery</span><span class="p">(</span>"<span class="n">Insert</span> <span class="n">into</span> <span class="n">TEST_TABLE</span> <span class="n">values</span> <span class="p">(</span> <span class="s">'Hello World!'</span> <span class="p">)</span>"<span class="p">));</span> |
| <span class="p">}</span> <span class="k">catch</span> <span class="p">(</span><span class="n">ScopedWorkException</span> <span class="n">swe</span><span class="p">)</span> <span class="p">{</span> |
| <span class="o">//</span> <span class="n">This</span> <span class="n">line</span> <span class="n">throws</span> <span class="n">the</span> <span class="n">cause</span> <span class="n">of</span> <span class="n">the</span> <span class="n">ScopedWorkException</span> <span class="n">as</span> |
| <span class="o">//</span> <span class="n">an</span> <span class="n">SQLException</span> <span class="n">or</span> <span class="n">as</span> <span class="n">a</span> <span class="n">RuntimeException</span> <span class="k">if</span> <span class="n">appropriate</span> |
| <span class="n">throw</span> <span class="n">swe</span><span class="p">.</span><span class="n">as</span><span class="p">(</span><span class="n">SQLException</span><span class="p">.</span><span class="n">class</span><span class="p">);</span> |
| <span class="p">}</span> |
| </pre></div> |
| |
| |
| <p>This mechanism also supports multiple Exception types:</p> |
| <div class="codehilite"><pre><span class="k">try</span> <span class="p">{</span> |
| <span class="n">txControl</span><span class="p">.</span><span class="n">required</span><span class="p">(()</span> <span class="o">-></span> <span class="n">connection</span><span class="p">.</span><span class="n">createStatement</span><span class="p">()</span> |
| <span class="p">.</span><span class="n">executeQuery</span><span class="p">(</span>"<span class="n">Insert</span> <span class="n">into</span> <span class="n">TEST_TABLE</span> <span class="n">values</span> <span class="p">(</span> <span class="s">'Hello World!'</span> <span class="p">)</span>"<span class="p">));</span> |
| <span class="p">}</span> <span class="k">catch</span> <span class="p">(</span><span class="n">ScopedWorkException</span> <span class="n">swe</span><span class="p">)</span> <span class="p">{</span> |
| <span class="o">//</span> <span class="n">This</span> <span class="n">line</span> <span class="n">throws</span> <span class="n">the</span> <span class="n">cause</span> <span class="n">of</span> <span class="n">the</span> <span class="n">ScopedWorkException</span> |
| <span class="o">//</span> <span class="n">as</span> <span class="n">one</span> <span class="n">of</span> <span class="n">the</span> <span class="n">two</span> <span class="n">SQLException</span> <span class="n">types</span> <span class="n">or</span> <span class="n">as</span> <span class="n">a</span> |
| <span class="o">//</span> <span class="n">RuntimeException</span> <span class="k">if</span> <span class="n">appropriate</span> |
| <span class="n">throw</span> <span class="n">swe</span><span class="p">.</span><span class="n">asOneOf</span><span class="p">(</span><span class="n">SQLRecoverableException</span><span class="p">.</span><span class="n">class</span><span class="p">,</span> <span class="n">SQLTransientException</span><span class="p">.</span><span class="n">class</span><span class="p">);</span> |
| <span class="p">}</span> |
| </pre></div> |
| |
| |
| <p>Note that if you unwrap a <code>ScopedWorkException</code> into a checked exception then you will have |
| to list that Exception in your <code>throws</code> clause.</p></div> |
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