在使用ConcurrentHashMap时,我发现computeIfAbsent方法的速度是putIfAbsent方法的两倍。这里是一个简单的测试:
import java.util.ArrayList;
import java.util.List;
import java.util.UUID;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
public class Test {
public static void main(String[] args) throws Exception {
String[] keys = {"a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9", "a0", "a01", "a02", "a03", "a04", "a05", "a06", "a07", "a08", "a09", "a00"};
System.out.println("Test case 1");
long time = System.currentTimeMillis();
testCase1(keys);
System.out.println("ExecutionTime: " + String.valueOf(System.currentTimeMillis() - time));
System.out.println("Test case 2");
time = System.currentTimeMillis();
testCase2(keys);
System.out.println("ExecutionTime: " + String.valueOf(System.currentTimeMillis() - time));
System.out.println("Test case 3");
time = System.currentTimeMillis();
testCase3(keys);
System.out.println("ExecutionTime: " + String.valueOf(System.currentTimeMillis() - time));
}
public static void testCase1(String[] keys) throws InterruptedException {
ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
List<Thread> threads = new ArrayList<>();
for (String key : keys) {
Thread thread = new Thread(() -> map.computeIfAbsent(key, s -> {
System.out.println(key);
String result = new TestRun().compute();
System.out.println("Computing finished for " + key);
return result;
}));
thread.start();
threads.add(thread);
}
for (Thread thread : threads) {
thread.join();
}
}
public static void testCase2(String[] keys) throws InterruptedException {
List<Thread> threads = new ArrayList<>();
for (String key : keys) {
Thread thread = new Thread(() -> {
System.out.println(key);
new TestRun().compute();
System.out.println("Computing finished for " + key);
});
thread.start();
threads.add(thread);
}
for (Thread thread : threads) {
thread.join();
}
}
public static void testCase3(String[] keys) throws InterruptedException {
ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();
List<Thread> threads = new ArrayList<>();
for (String key : keys) {
Thread thread = new Thread(() -> {
Callable<String> c = () -> {
System.out.println(key);
String result = new TestRun().compute();
System.out.println("Computing finished for " + key);
return result;
};
try {
map.putIfAbsent(key, c.call());
} catch (Exception e) {
e.printStackTrace(System.out);
}
});
thread.start();
threads.add(thread);
}
for (Thread thread : threads) {
thread.join();
}
}
}
class TestRun {
public String compute() {
try {
Thread.currentThread().sleep(5000);
} catch (Exception e) {
e.printStackTrace(System.out);
}
return UUID.randomUUID().toString();
}
}
在我的笔记本电脑上运行此测试,使用computeIfAbsent()的testCase1执行时间为10068ms,而在未将其封装到computeIfAbsent()中执行相同操作的testCase2执行时间为5009ms(当然会有些许变化,但主要趋势是如此)。最有趣的是testCase3——它与testCase1非常相似(除了使用putIfAbsent()代替computeIfAbsent()),但其执行速度快两倍(testCase3的执行时间为5010ms,而testCase1为10068ms)。
从源代码来看,computeIfAbsent()和putVal()(在putIfAbsent()中使用)基本相同。
有谁知道是什么导致线程执行时间不同吗?
putIfAbsent()
已经有了对象。computeIfAbsent()
必须执行一个方法来确定对象。这都有文档记录。 - user207421Thread.sleep(5000)
来很好地展示ConcurrentHashMap
的一个特定行为。至少在这个问题上,没有必要进行适当的基准测试。 - Luke Usherwood