其他
Java变量声明在循环体内还是循环体外，你用哪一个？
The following article is from java金融 Author java金融
来源 | java金融（ID：java4299）头图 | CSDN 下载自东方IC
引言
for (int i = 1; i <= count; i++){ Object obj = new Object();}Object obj = null;for (int i = 0; i <= count; i++) { obj = new Object();}效率对比
/** * @author: 公众号【java金融】 * @Date: * @Description: */
@BenchmarkMode(Mode.AverageTime) // 测试完成时间@OutputTimeUnit(TimeUnit.NANOSECONDS)@Warmup(iterations = 2) // 预热 2 轮，每次 1s@Measurement(iterations = 5) // 测试 5 轮，每次 1s@Fork(1) // fork 1 个线程@State(Scope.Thread)public class ForEachBenchMark {
public static void main(String[] args) throws RunnerException { Options opt = new OptionsBuilder() .include(ForEachBenchMark.class.getSimpleName()) .result("result.json") .resultFormat(ResultFormatType.JSON).build(); new Runner(opt).run(); }
@Param(value = {"10", "50", "100"}) private int length;
/** * 循环体外创建对象 * @param blackhole */ @Benchmark public void outsideLoop(Blackhole blackhole) { Object object = null; for (int i = 0; i < length; i++) { object = new Object(); blackhole.consume(object); }
}
/** * 循环体内创建对象 * @param blackhole */ @Benchmark public void insideLoop(Blackhole blackhole) { for (int i = 0; i < length; i++) { Object object = new Object(); blackhole.consume(object);
}
}}Benchmark (length) Mode Cnt Score Error Units
ForEachBenchMark.insideLoop 10 avgt 5 58.629 ± 8.857 ns/op
ForEachBenchMark.insideLoop 50 avgt 5 293.726 ± 1.856 ns/op
ForEachBenchMark.insideLoop 100 avgt 5 587.185 ± 40.424 ns/op
ForEachBenchMark.outsideLoop 10 avgt 5 59.563 ± 5.057 ns/op
ForEachBenchMark.outsideLoop 50 avgt 5 305.829 ± 27.476 ns/op
ForEachBenchMark.outsideLoop 100 avgt 5 584.853 ± 20.289 ns/op
字节码对比
public class InsideTest { public static int count = 100; public List<Object> insideLoop() { List<Object> list = new ArrayList<>(); int n = 0; for (; ; ) { if (n > count) { break; } Object o = new Object(); list.add(o); } Object b = 2; return list; }}public class OutsideTest { public static int count = 100;public List<Object> outsideLoop() { List<Object> list = new ArrayList<>(); Object o = null; int n = 0; for (; ; ) { if (n > count) { break; } o = new Object(); list.add(o); } Object b = 2; return list; }javac -g OutsideTest.javajavap -v OutsideTest.class LocalVariableTable: Start Length Slot Name Signature 28 8 3 o Ljava/lang/Object; 0 46 0 this Lcom/workit/autoconfigure/autoconfigure/controller/InsideTest; 8 38 1 list Ljava/util/List; 10 36 2 n I 44 2 3 b Ljava/lang/Object;LocalVariableTable: Start Length Slot Name Signature 0 49 0 this Lcom/workit/autoconfigure/autoconfigure/controller/OutsideTest; 8 41 1 list Ljava/util/List; 10 39 2 o Ljava/lang/Object; 12 37 3 n I 47 2 4 b Ljava/lang/Object;总结
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