java - Java Concurrency in Practice 中 ImprovedList 的线程安全

Question

我阅读了 Java 并发实践。它说这个类是线程安全的:

package net.jcip.examples;

import java.util.*;

import net.jcip.annotations.*;

/**
 * ImprovedList
 *
 * Implementing put-if-absent using composition
 *
 * @author Brian Goetz and Tim Peierls
 */
@ThreadSafe
public class ImprovedList<T> implements List<T> {
    private final List<T> list;

    public ImprovedList(List<T> list) { this.list = list; }

    public synchronized boolean putIfAbsent(T x) {
        boolean contains = list.contains(x);
        if (contains)
            list.add(x);
        return !contains;
    }

    // Plain vanilla delegation for List methods.
    // Mutative methods must be synchronized to ensure atomicity of putIfAbsent.

    public int size() {
        return list.size();
    }

    public boolean isEmpty() {
        return list.isEmpty();
    }

    public boolean contains(Object o) {
        return list.contains(o);
    }

    public Iterator<T> iterator() {
        return list.iterator();
    }

    public Object[] toArray() {
        return list.toArray();
    }

    public <T> T[] toArray(T[] a) {
        return list.toArray(a);
    }

    public synchronized boolean add(T e) {
        return list.add(e);
    }

    public synchronized boolean remove(Object o) {
        return list.remove(o);
    }

    public boolean containsAll(Collection<?> c) {
        return list.containsAll(c);
    }

    public synchronized boolean addAll(Collection<? extends T> c) {
        return list.addAll(c);
    }

    public synchronized boolean addAll(int index, Collection<? extends T> c) {
        return list.addAll(index, c);
    }

    public synchronized boolean removeAll(Collection<?> c) {
        return list.removeAll(c);
    }

    public synchronized boolean retainAll(Collection<?> c) {
        return list.retainAll(c);
    }

    public boolean equals(Object o) {
        return list.equals(o);
    }

    public int hashCode() {
        return list.hashCode();
    }

    public T get(int index) {
        return list.get(index);
    }

    public T set(int index, T element) {
        return list.set(index, element);
    }

    public void add(int index, T element) {
        list.add(index, element);
    }

    public T remove(int index) {
        return list.remove(index);
    }

    public int indexOf(Object o) {
        return list.indexOf(o);
    }

    public int lastIndexOf(Object o) {
        return list.lastIndexOf(o);
    }

    public ListIterator<T> listIterator() {
        return list.listIterator();
    }

    public ListIterator<T> listIterator(int index) {
        return list.listIterator(index);
    }

    public List<T> subList(int fromIndex, int toIndex) {
        return list.subList(fromIndex, toIndex);
    }

    public synchronized void clear() { list.clear(); }
}

我不这么认为。可以在另一个线程中引用构造函数参数 list。所以如果这个 list 不是线程安全的，它可能在并发性上不一致。如果这个list是线程安全的，另一个线程用list的锁操作它，使用ImprovedList的线程用ImprovedList的锁操作它对象锁，所以他们仍然可以同时改变它。

谁能告诉我哪里错了？

Answer 1

本书引文:

this is guaranteed to provide thread safety so long as our class holds the only outstanding reference to the underlying List.

所以，如果有人持有对同一列表的引用，则该类不能称为线程安全的，这是正确的。但是你不专心看书就错了。