285 lines
11 KiB
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Generated
285 lines
11 KiB
Plaintext
Generated
<!DOCTYPE html>
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<meta charset="utf-8">
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<title>Go by Example: Recover</title>
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<div class="example" id="recover">
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<h2><a href="./">Go by Example</a>: Recover</h2>
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<table>
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<td class="docs">
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<p>A <code>recover</code> means recovering from a <code>panic</code>, either from a “business” or “built-in” panic.\n
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We want to recover if we want to handle a panic, stopping it from propagating upwards.</p>
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<a href="http://play.golang.org/p/uVo4c0IE97q"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
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<span class="s">"fmt"</span>
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<span class="p">)</span>
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</pre></div>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
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</pre></div>
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<div class="highlight"><pre> <span class="nx">recoverFromBuiltInPanic</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
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</pre></div>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">()</span>
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</pre></div>
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<div class="highlight"><pre> <span class="nx">recoverFromCustomPanic</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span>
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<span class="p">}</span>
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</pre></div>
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<td class="docs">
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<p>defer is defined.</p>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">recoverFromBuiltInPanic</span><span class="p">(</span><span class="nx">i</span> <span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
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</pre></div>
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<div class="highlight"><pre> <span class="k">defer</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">if</span> <span class="nx">r</span> <span class="o">:=</span> <span class="nb">recover</span><span class="p">();</span> <span class="nx">r</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"Recovered. Error:\n"</span><span class="p">,</span> <span class="nx">r</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="p">}()</span>
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</pre></div>
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<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">a</span> <span class="p">[</span><span class="mi">5</span><span class="p">]</span><span class="kt">int</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">"Getting index %d"</span><span class="o">+</span>
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<span class="s">" of array of len %d...\n"</span><span class="p">,</span> <span class="nx">i</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="nx">a</span><span class="p">))</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">"Item in index %d: %d"</span><span class="p">,</span> <span class="nx">i</span><span class="p">,</span> <span class="nx">a</span><span class="p">[</span><span class="nx">i</span><span class="p">])</span>
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<span class="p">}</span>
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</pre></div>
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<td class="docs">
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<p>defer is defined.</p>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">recoverFromCustomPanic</span><span class="p">(</span><span class="nx">i</span> <span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
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</pre></div>
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<div class="highlight"><pre> <span class="k">defer</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
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<span class="k">if</span> <span class="nx">r</span> <span class="o">:=</span> <span class="nb">recover</span><span class="p">();</span> <span class="nx">r</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"Recovered. Error:\n"</span><span class="p">,</span> <span class="nx">r</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="p">}()</span>
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</pre></div>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">"About to process i=%d\n"</span><span class="p">,</span> <span class="nx">i</span><span class="p">)</span>
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</pre></div>
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<div class="highlight"><pre> <span class="k">if</span> <span class="nx">i</span> <span class="p"><</span> <span class="mi">0</span> <span class="p">{</span>
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<span class="nb">panic</span><span class="p">(</span><span class="nx">fmt</span><span class="p">.</span><span class="nx">Errorf</span><span class="p">(</span><span class="s">"Accepting only"</span><span class="o">+</span>
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<span class="s">" non-negative numbers but received %d"</span><span class="p">,</span> <span class="nx">i</span><span class="p">))</span>
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<span class="p">}</span>
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</pre></div>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">"Doing something with %d\n"</span><span class="p">,</span> <span class="nx">i</span><span class="p">)</span>
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<span class="p">}</span>
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</pre></div>
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</table>
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<table>
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<p>Running this program will exit correctly,
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even though panic was invoked in two methods.
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The recover is responsible for recovering from panics.</p>
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<div class="highlight"><pre><span class="gp">$</span> go run recover.go
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<span class="go">Getting index 10 of array of len 5...</span>
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<span class="go">Recovered. Error:</span>
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<span class="go"> runtime error: index out of range [10] with length 5</span>
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</pre></div>
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<td class="docs">
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<p>Note that, in Go it is idiomatic
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to use error-indicating return values wherever possible.</p>
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<td class="code">
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<div class="highlight"><pre><span class="go">About to process i=-1</span>
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<span class="go">Recovered. Error:</span>
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<span class="go"> Accepting only non-negative numbers but received -1</span>
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</pre></div>
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</table>
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<p class="next">
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Next example: <a href="collection-functions">Collection Functions</a>.
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</p>
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<p class="footer">
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/recover">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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</p>
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</div>
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<script>
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var codeLines = [];
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codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push(' recoverFromBuiltInPanic(10)\u000A');codeLines.push(' fmt.Println()\u000A');codeLines.push(' recoverFromCustomPanic(-1)\u000A}\u000A');codeLines.push('func recoverFromBuiltInPanic(i int) {\u000A');codeLines.push(' defer func() {\u000A if r :\u003D recover(); r !\u003D nil {\u000A fmt.Println(\"Recovered. Error:\\n\", r)\u000A }\u000A }()\u000A');codeLines.push(' var a [5]int\u000A fmt.Printf(\"Getting index %d\"+\u000A \" of array of len %d...\\n\", i, len(a))\u000A fmt.Printf(\"Item in index %d: %d\", i, a[i])\u000A}\u000A');codeLines.push('func recoverFromCustomPanic(i int) {\u000A');codeLines.push(' defer func() {\u000A if r :\u003D recover(); r !\u003D nil {\u000A fmt.Println(\"Recovered. Error:\\n\", r)\u000A }\u000A }()\u000A');codeLines.push(' fmt.Printf(\"About to process i\u003D%d\\n\", i)\u000A');codeLines.push(' if i \u003C 0 {\u000A panic(fmt.Errorf(\"Accepting only\"+\u000A \" non-negative numbers but received %d\", i))\u000A }\u000A');codeLines.push(' fmt.Printf(\"Doing something with %d\\n\", i)\u000A}\u000A');codeLines.push('');codeLines.push('');
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