给定以下代码片段,该代码片段生成UUID.randomUUID()
,我得到以下性能结果(以毫秒为单位):
public static void main(String[] args) {
long tmp = System.currentTimeMillis();
UUID.randomUUID();
tmp = printDiff(tmp);
UUID.randomUUID();
tmp = printDiff(tmp);
UUID.randomUUID();
tmp = printDiff(tmp);
UUID.randomUUID();
tmp = printDiff(tmp);
}
private static long printDiff(final long previousTimestamp) {
long tmp = System.currentTimeMillis();
System.out.printf("%s%n", tmp - previousTimestamp);
return tmp;
}
结果:
971
6
0
0
JDK:1.8操作系统:视窗 7
为什么只有初始调用需要这么长时间?(近1秒!
SecureRandom 的初始化只完成一次:
//from the source code of randomUUID
private static class Holder {
static final SecureRandom numberGenerator = new SecureRandom();
}
但这还不是全部。那些零真的应该跳到你的脸上。所以操作花了 0 毫秒;这是否意味着他们拿的更少?比如几纳秒,或者你做错了什么?
有一个合适的工具来测量这些东西,叫做jmh。
@BenchmarkMode({ Mode.AverageTime, Mode.SingleShotTime })
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@Warmup(iterations = 2, time = 2, timeUnit = TimeUnit.SECONDS)
@Measurement(iterations = 2, time = 2, timeUnit = TimeUnit.SECONDS)
@State(Scope.Benchmark)
public class UUIDRandom {
public static void main(String[] args) throws RunnerException {
Options opt = new OptionsBuilder().include(UUIDRandom.class.getSimpleName()).build();
new Runner(opt).run();
}
@Benchmark
@Fork(1)
public UUID random() {
return UUID.randomUUID();
}
}
输出说:
Benchmark Mode Cnt Score Error Units
UUIDRandom.random avgt 2 0.002 ms/op
UUIDRandom.random ss 2 0.094 ms/op
事实上,单次拍摄时间远低于平均水平。
调用UUID.randomUUID()
时,它必须初始化一些内部对象,这些对象也在所有后续调用中使用。
UUID.randomUUID
的源代码是:
public static UUID randomUUID() {
SecureRandom ng = Holder.numberGenerator;
byte[] randomBytes = new byte[16];
ng.nextBytes(randomBytes);
randomBytes[6] &= 0x0f; /* clear version */
randomBytes[6] |= 0x40; /* set to version 4 */
randomBytes[8] &= 0x3f; /* clear variant */
randomBytes[8] |= 0x80; /* set to IETF variant */
return new UUID(randomBytes);
}
在这里,Holder.numberGenerator
是一个全局变量,在第一次使用时必须初始化:
private static class Holder {
static final SecureRandom numberGenerator = new SecureRandom();
}
根据 Java 8 代码,创建 SecureRandom
对象似乎很昂贵。这就是为什么它们将初始化推迟到需要(也称为延迟初始化)并在后续调用中重用它的原因。
/*
* The random number generator used by this class to create random
* based UUIDs. In a holder class to defer initialization until needed.
*/
private static class Holder {
static final SecureRandom numberGenerator = new SecureRandom();
}