最近使用mvel 2.2.0.Final，出现一次cpu跑满，经过线程栈分析，发现是误用WeakHashMap引起的。
故障现场：
看WeakHashMap源码：
public V get(Object key) {
Object k = maskNull(key);
int h = hash(k);
Entry<K,V>[] tab = getTable();
int index = indexFor(h, tab.length);
Entry<K,V> e = tab[index];
while (e != null) {
if (e.hash == h && eq(k, e.get()))
return e.value;
e = e.next;
}
return null;
}线程在WeakHashMap的get方法里面出不来了，一直在while循环里面。
多线程并发get和put，fullgc或gc的时候可能会出现。
因为gc会把对象给清理掉，然后get方法内的while循环一直找不到eq的对象，循环出不来。
WeakHashMap类已经说明了这个类不是线程安全的。在[2.1.8.Final,~]以上修复了，除了2.2.0.Final，修复详情。
问题复现：
import java.util.Map;
import java.util.Random;
import java.util.WeakHashMap; public class WeakHashMapTest { Thread thread = new Thread(new Runnable() { @Override
public void run() {
}
}); public static void main(String[] args) throws InterruptedException {
Random random = new Random();
// Map<String, String> weak = Collections.synchronizedMap(new WeakHashMap<>());//OK
Map<String, String> weak = new WeakHashMap<>();
for (int i = 0; i < 10; i++) {
weak.put(new String("" + i), "" + i);
}
for (int i = 0; i < 20; i++) {
new Thread(new Runnable() { @Override
public void run() {
StringBuffer sb = new StringBuffer();
for (int k = 0; k < 200; k++) {
sb.append(weak.get(new String("" + (k) % 10)));
if (k % 17 == 0) {
System.gc();
}
int nextInt = random.nextInt(10);
weak.put(new String("" + nextInt), "" + nextInt);
}
System.out.println("end:" + sb.toString());
}
}).start();
}
System.gc();
System.out.println("sleep");
Thread.sleep(10000);
System.out.println("exit");
System.out.println("exit2");
} static void test1() {
Map<String, String> weak = new WeakHashMap<>();
weak.put(new String("1"), "1");
weak.put(new String("2"), "2");
weak.put(new String("3"), "3");
weak.put(new String("4"), "4");
weak.put(new String("5"), "5");
weak.put(new String("6"), "6");
System.out.println(weak.size());
System.gc(); //手动触发 Full GC
try {
Thread.sleep(50); //我的测试中发现必须sleep一下才能看到不一样的结果
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(weak.size());
} }