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copied!<p>The Hotspot JIT only inlines methods that are less than a certain (configurable) size. So using smaller methods allows more inlining, which is good.</p> <p>See the various inlining options on <a href="http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html" rel="noreferrer">this page</a>.</p> <hr> <p><strong>EDIT</strong></p> <p>To elaborate a little:</p> <ul> <li>if a method is small it will get inlined so there is little chance to get penalised for splitting the code in small methods.</li> <li>in some instances, splitting methods may result in more inlining.</li> </ul> <p><strong>Example</strong> (full code to have the same line numbers if you try it)</p> <pre><code>package javaapplication27; public class TestInline { private int count = 0; public static void main(String[] args) throws Exception { TestInline t = new TestInline(); int sum = 0; for (int i = 0; i < 1000000; i++) { sum += t.m(); } System.out.println(sum); } public int m() { int i = count; if (i % 10 == 0) { i += 1; } else if (i % 10 == 1) { i += 2; } else if (i % 10 == 2) { i += 3; } i += count; i *= count; i++; return i; } } </code></pre> <p>When running this code with the following JVM flags: <code>-XX:+UnlockDiagnosticVMOptions -XX:+PrintCompilation -XX:FreqInlineSize=50 -XX:MaxInlineSize=50 -XX:+PrintInlining</code> (yes I have used values that prove my case: <code>m</code> is too big but both the refactored <code>m</code> and <code>m2</code> are below the threshold - with other values you might get a different output).</p> <p>You will see that <code>m()</code> and <code>main()</code> get compiled, but <code>m()</code> does not get inlined:</p> <pre><code> 56 1 javaapplication27.TestInline::m (62 bytes) 57 1 % javaapplication27.TestInline::main @ 12 (53 bytes) @ 20 javaapplication27.TestInline::m (62 bytes) too big </code></pre> <p>You can also inspect the generated assembly to confirm that <code>m</code> is not inlined (I used these JVM flags: <code>-XX:+PrintAssembly -XX:PrintAssemblyOptions=intel</code>) - it will look like this:</p> <pre><code>0x0000000002780624: int3 ;*invokevirtual m ; - javaapplication27.TestInline::main@20 (line 10) </code></pre> <p>If you refactor the code like this (I have extracted the if/else in a separate method):</p> <pre><code>public int m() { int i = count; i = m2(i); i += count; i *= count; i++; return i; } public int m2(int i) { if (i % 10 == 0) { i += 1; } else if (i % 10 == 1) { i += 2; } else if (i % 10 == 2) { i += 3; } return i; } </code></pre> <p>You will see the following compilation actions:</p> <pre><code> 60 1 javaapplication27.TestInline::m (30 bytes) 60 2 javaapplication27.TestInline::m2 (40 bytes) @ 7 javaapplication27.TestInline::m2 (40 bytes) inline (hot) 63 1 % javaapplication27.TestInline::main @ 12 (53 bytes) @ 20 javaapplication27.TestInline::m (30 bytes) inline (hot) @ 7 javaapplication27.TestInline::m2 (40 bytes) inline (hot) </code></pre> <p>So <code>m2</code> gets inlined into <code>m</code>, which you would expect so we are back to the original scenario. But when <code>main</code> gets compiled, it actually inlines the whole thing. At the assembly level, it means you won't find any <code>invokevirtual</code> instructions any more. You will find lines like this:</p> <pre><code> 0x00000000026d0121: add ecx,edi ;*iinc ; - javaapplication27.TestInline::m2@7 (line 33) ; - javaapplication27.TestInline::m@7 (line 24) ; - javaapplication27.TestInline::main@20 (line 10) </code></pre> <p>where basically common instructions are "mutualised".</p> <p><strong>Conclusion</strong></p> <p>I am not saying that this example is representative but it seems to prove a few points:</p> <ul> <li>using smaller method improves readability in your code</li> <li>smaller methods will generally be inlined, so you will most likely not pay the cost of the extra method call (it will be performance neutral)</li> <li>using smaller methods <em>might</em> improve inlining globally in some circumstances, as shown by the example above</li> </ul> <p>And finally: if a portion of your code is really critical for performance that these considerations matter, you should examine the JIT output to fine tune your code and importantly profile before and after.</p>

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