final Map<Integer, String> original = new HashMap<Integer, String>();
final int capacity = 1000 * 100;
for (int i = 0; i < capacity; i++) {
original.put(i, UUID.randomUUID().toString());
}
final Map<Integer, String> unmodifiable = Collections.unmodifiableMap(original);
ExecutorService executor = Executors.newFixedThreadPool(2);
for (int k = 0; k < 5; k++) {
executor.execute(new Runnable() {
@Override
public void run() {
Random r = new Random();
int c = original.size();
for (int i = c; i < c + 1000; i++) {
original.put(i, UUID.randomUUID().toString());
}
}
});
executor.execute(new Runnable() {
@Override
public void run() {
StringBuilder sb = new StringBuilder();
for (Map.Entry<Integer, String> entry : unmodifiable.entrySet()) {
sb.append(entry.getValue()).append(' ');
}
System.out.println("sb.toString().length() = " + sb.toString().length());
}
});
}
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
public class TestUnmod {
public static void main(String[] args) {
Map<String,String> map = new HashMap();
map.put("a", "a");
map.put("b", "b");
map.put("c", "c");
Map<String,String> unmod = Collections.unmodifiableMap(map);
Iterator<String> it = unmod.values().iterator();
System.out.println(it.next());
map.put("d", "d");
System.out.println(it.next());
}
}
输出:
b
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
at java.util.HashMap$ValueIterator.next(HashMap.java:822)
at java.util.Collections$UnmodifiableCollection$1.next(Collections.java:1010)
at TestUnmod.main(TestUnmod.java:18)
# 1 楼答案
下面是一个更新的示例,显示抛出了异常(谢谢@jarnbjo):
# 2 楼答案
不,它们不会很快失败
虽然没有特别提到,但API文档说明“返回映射上的查询操作”读取“指定映射”,并尝试修改返回的映射…,会导致“UnsupportedOperationException”。由于不可修改的映射仅委托给原始映射,因此有关使用原始映射的所有限制也与不可修改包装器相关
# 3 楼答案
由于API没有定义行为,因此它依赖于实现,也就是说,它可能会根据您使用的Java运行时而有所不同
对于Sun Java 6运行时,其行为似乎将从正在包装的映射中继承。不过,正如我所说,这是明智的行为,并不能保证跨实现
编辑-Sun Java 6的测试用例:
输出: