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<td class="text-paragraph-pattern-container mobile-resize-text " style="padding: 0px; border-collapse: collapse; padding: 0 0 10px 0"> <p style="margin: 10px 0 0 0; margin-top: 0"><a href="https://github.com/huxi/lilith/commit/e109c37335b8737ef69b821173263ede8b5850c3" class="external-link" rel="nofollow" style="color: #3b73af; text-decoration: none">This commit</a> implements your suggested fix.</p> <p style="margin: 10px 0 0 0">The relevant code looks like this:</p>
<div class="code panel" style="border-width: 1px;; border: 1px solid #cccccc; background: #f5f5f5; font-size: 12px; line-height: 1.333; font-family: monospace; border: 1px solid #cccccc; -moz-border-radius: 3px 3px 3px 3px; border-radius: 3px 3px 3px 3px; margin: 9px 0">
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<pre class="code-java" style="margin: 10px 0 0 0; margin-top: 0; max-height: 30em; overflow: auto; white-space: pre-wrap; word-wrap: normal">
<span class="code-comment" style="color: #808080">// suggested LOGBACK-1257 workaround below
</span><span class="code-keyword" style="color: #000091">private</span> OutputStreamAppender<?> parent;
<span class="code-keyword" style="color: #000091">public</span> void setParent(OutputStreamAppender<?> parent)
{
<span class="code-keyword" style="color: #000091">this</span>.parent = parent;
}
<span class="code-keyword" style="color: #000091">private</span> <span class="code-object" style="color: #910091">boolean</span> isSupposedToGenerateHeader()
{
<span class="code-keyword" style="color: #000091">if</span>(parent <span class="code-keyword" style="color: #000091">instanceof</span> FileAppender)
{
FileAppender fileAppender = (FileAppender) parent;
<span class="code-keyword" style="color: #000091">if</span>(!fileAppender.isAppend())
{
<span class="code-keyword" style="color: #000091">return</span> <span class="code-keyword" style="color: #000091">true</span>;
}
OutputStream outputStream = fileAppender.getOutputStream();
<span class="code-keyword" style="color: #000091">if</span>(outputStream <span class="code-keyword" style="color: #000091">instanceof</span> ResilientFileOutputStream)
{
ResilientFileOutputStream fileOutputStream = (ResilientFileOutputStream) outputStream;
File file = fileOutputStream.getFile();
<span class="code-keyword" style="color: #000091">if</span>(file.length() != 0)
{
<span class="code-comment" style="color: #808080">// appending and file with already existing content => no header.
</span> <span class="code-keyword" style="color: #000091">return</span> <span class="code-keyword" style="color: #000091">false</span>;
}
}
}
<span class="code-keyword" style="color: #000091">return</span> <span class="code-keyword" style="color: #000091">true</span>;
}
<span class="code-comment" style="color: #808080">// not necessary <span class="code-keyword" style="color: #000091; color: #808080">for</span> <span class="code-keyword" style="color: #000091; color: #808080">this</span> encoder since it doesn't require a footer
</span>/*
<span class="code-keyword" style="color: #000091">private</span> <span class="code-object" style="color: #910091">boolean</span> isSupposedToGenerateFooter()
{
<span class="code-keyword" style="color: #000091">if</span>(parent <span class="code-keyword" style="color: #000091">instanceof</span> FileAppender)
{
FileAppender fileAppender = (FileAppender) parent;
<span class="code-keyword" style="color: #000091">if</span>(fileAppender.isAppend())
{
<span class="code-keyword" style="color: #000091">return</span> <span class="code-keyword" style="color: #000091">false</span>;
}
}
<span class="code-keyword" style="color: #000091">return</span> <span class="code-keyword" style="color: #000091">true</span>;
}
*/
<span class="code-comment" style="color: #808080">// suggested LOGBACK-1257 workaround above</span>
</pre>
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</div> <p style="margin: 10px 0 0 0">The <tt>headerBytes()</tt> is returning <tt>null</tt> if <tt>!isSupposedToGenerateHeader()</tt> while <tt>footerBytes()</tt> would return <tt>null</tt> if <tt>!isSupposedToGenerateFooter()</tt> (not necessary in my case because I don't need/have a footer).</p> <p style="margin: 10px 0 0 0">This <em>works</em> but I <b>wholeheartedly disagree</b> that this is the way this should be implemented for several reasons:</p>
<ul>
<li>Every single <tt>Encoder</tt> in need of header or footer would have to re-implement this identical functionality.</li>
<li>The logic above is non-trivial to test which is a good indicator that something is wrong about the approach. Testing something simple like an <tt>Encoder</tt> should be entirely straight-forward.</li>
<li><b>It should be the responsibility of <tt>Appender</tt> implementations to know whether or not or in which cases headers or footers should be written.</b> If someone decided to implement a <tt>TapeDriveAppender</tt> unrelated to <tt>FileAppender</tt>, the above logic would have to be expanded accordingly, for each and every new <tt>Appender</tt> implementation. Each special handling would further increase the complexity of the <tt>Encoder</tt> tests.</li>
<li><a href="https://en.wikipedia.org/wiki/Single_responsibility_principle" class="external-link" rel="nofollow" style="color: #3b73af; text-decoration: none">Single responsibility principle</a> and <a href="https://en.wikipedia.org/wiki/Dependency_inversion_principle" class="external-link" rel="nofollow" style="color: #3b73af; text-decoration: none">Dependency inversion principle</a> are violated by the above approach.</li>
<li>Because of this, the above code is very fragile in nature and likely to break in case of otherwise internal changes to Logback. It would break if <tt>ch.qos.logback.core.FileAppender</tt>, <tt>ch.qos.logback.core.OutputStreamAppender</tt>, <tt>ch.qos.logback.core.recovery.ResilientFileOutputStream</tt> or <tt>ch.qos.logback.core.encoder.EncoderBase</tt> changed in an incompatible way while only <tt>ch.qos.logback.core.encoder.EncoderBase</tt> should have any impact.</li>
<li>The above mentioned fragility isn't a theoretical issue either because the tests of the previous workaround implementation already broke when you replaced <tt>ResilientFileOutputStream(File file, boolean append)</tt> with <tt>ResilientFileOutputStream(File file, boolean append, long bufferSize)</tt> between <tt>1.1.9</tt> and <tt>1.1.10</tt> releases. See <a href="https://github.com/huxi/lilith/commit/2c6c32bba58ee705edbdbb0bfe05422b953c7818" class="external-link" rel="nofollow" style="color: #3b73af; text-decoration: none">the related commit</a>.</li>
</ul> <p style="margin: 10px 0 0 0">While I implemented the above workaround, I was taking a look at <tt>ch.qos.logback.classic.log4j.XMLLayout</tt> because surely some similar logic would have to be present in that class.</p> <p style="margin: 10px 0 0 0">Well... it isn't.</p> <p style="margin: 10px 0 0 0">The reason for this is that <tt>XMLLayout</tt> does <b>not</b> (in contrast to what the javadoc is saying) write XML documents compliant to <a href="http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/xml/doc-files/log4j.dtd" class="external-link" rel="nofollow" style="color: #3b73af; text-decoration: none">log4j.dtd</a>. It is omitting the (optional) <tt><?xml version="1.0" encoding="UTF-8"?></tt> XML prolog as well as the (mandatory) enclosing <tt><eventSet></eventSet></tt> root element, instead simply concatenating <tt><event></event></tt> elements. At that point I remembered the workaround I implemented years ago to parse those files.</p> <p style="margin: 10px 0 0 0">A proper implementation (i.e. one that is creating well-formed XML documents compliant to the DTD) of an <tt>XMLEncoder</tt> would need the exact same logic implemented in the above workaround to decide if header and footers should be written.</p> </td>
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