# 在现有的线程安全类中添加功能

## <mark style="color:blue;">**修改原始的类**</mark>

要在已有的线程安全类中添加一个新的原子操作，最安全的方法是**修改原始的类。**

**不足之处：**&#x8FD9;通常无法做到，因为可能无法访问或修改类的源代码。

## <mark style="color:blue;">**扩展类机制**</mark>

如果在设计这个线程安全类时考虑了可扩展性，那么可以通过**继承**来添加原子操作。

比如以下代码中的 BetterVector 扩展了 Vector 类，并添加了一个新的同步方法 putIfAbsent：

```java
public class BetterVector<E> extends Vector<E> {
    public synchronized boolean putIfAbsent(E e) {
        boolean absent = !contains(e);
        if (absent)
            add(e);
        return absent;
    }
}
```

**不足之处：**&#x5982;果父类改变了同步策略并选择了不同的锁来保护它的状态变量，那么子类会被破坏。

## <mark style="color:blue;">**客户端加锁机制**</mark>

扩展类的功能，但并不是扩展类本身，而是将扩展代码放入一个“辅助类”中。

如下所示：

```java
public class ListHelper<E> {
    public List<E> list = Collections.synchronizedList(new ArrayList<E>());
    ...
    public boolean putIfAbsent(E e) {
        synchronized(list) {
            boolean absent = !contains(e);
            if (absent)
                add(e);
            return absent;
        }
    }
}
```

**不足之处：客户端加锁机制与扩展类机制一样，将派生类的行为与基类的实现耦合在了一起。**&#x5982;果基类改变了同步策略并选择了不同的锁来保护它的状态变量，那么“辅助类”会被破坏。

## <mark style="color:blue;">**组合**</mark>

```java
public class ImprovedList<E> implements List<E> {
    private final List<E> list;
    
    public ImprovedList(List<E> list) {
        this.list = list;
    }
    
    public synchronized putIfAbsent(E e) {
        boolean absent = !list.contains(e);
        if (absent)
            list.add(e);
        return absent;
    }
    
    public synchronized void clear() { list.clear(); }
    ...
} 
```

ImprovedList 通过自身的内置锁增加了一层额外的加锁。它并不关心底层的 List 是否是线程安全的，即使 List 不是线程安全的或者修改了它的加锁实现，ImprovedList 也会提供一致的加锁机制来实现线程安全性。


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://bohans.gitbook.io/ji-chu/ji-chu-zhi-shi/java-bing-fa/xian-cheng/xian-cheng-an-quan-xing/zai-xian-you-de-xian-cheng-an-quan-lei-zhong-tian-jia-gong-neng.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.