常用的设计模式笔记

单例

这是一种简单、安全的方式，这种写法即使在多线程下也不会出现安全问题：

public class SingleInstance {
    private static SingleInstance instance = new SingleInstance();
    private SingleInstance() {}
    public static SingleInstance getInstance() {
        return instance;
    }
}

懒加载

public class SingleInstance {
    private static volatile SingleInstance instance;
    private SingleInstance() {}
    public static SingleInstance getInstance() {
        if (instance == null) { // 减少线程进入竞争队列
            synchronized (SingleInstance.class) {
                if (instance == null) { // 在多个线程进入竞争队列后，再次判断
                    instance = new SingleInstance();
                }
            }
        }
        return instance;
    }
}

责任链

比如 Tomcat 中的 Filter，递归责任链

public class CustomFilter implements Filter {
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) {
        preHandle();
        chain.doFilter(request, response);
        postHandle();
    }
}