定義

Strategy design pattern is one of the Gang of Four (GOF) design patterns. Strategy design pattern defines a set of encapsulated algorithms that can be swapped to carry out a specific behavior

一個功能，替換不同的實現(xiàn)方法；
功能 1.....N 實現(xiàn)方法

類圖

image.png

• Context: 用來操作策略的接口；
• Strategy: 策略接口
• ConcreteStrategy: 具體的策略；可進(jìn)行替換的就是這部分內(nèi)容
• Tip：策略的實現(xiàn)是一種多態(tài)的應(yīng)用，掌握多態(tài)是實現(xiàn)設(shè)計模式的關(guān)鍵。

適用場景

一個功能需要提供不同的實現(xiàn)方法的時候；
比如java中的compare可以寫一個自己的實現(xiàn)，來替換原有的compare；Person可以使用age來比較，也可以使用weight來進(jìn)行比較；

Java中的Comparator

import java.util.ArrayList;
import java.util.List;

/**
 * <p>
 * description
 * </P>
 *
 * @author: zhangss
 * @since: 2020-07-01
 **/
public class Person {
    public int age;
    public int weight;

    private Person(int age, int weight){
        this.age = age;
        this.weight = weight;
    }

    @Override
    public String toString() {
        return "age: " + age + " weight: " + weight;
    }

    public static void main(String[] args) {
        List<Person> personList = new ArrayList<>();
        personList.add(new Person(16, 120));
        personList.add(new Person(11, 54));
        personList.add(new Person(30, 100));

        System.out.println(personList);

        personList.sort((o1, o2)->{
            if(o1.age > o2.age){
                return 1;
            }
            return -1;
        });

        System.out.println("sort by age: " + personList);

        personList.sort((o1, o2)->{
            if(o1.weight > o2.weight){
                return 1;
            }
            return -1;
        });

        System.out.println("sort by weight: " + personList);
    }
}
// 運行結(jié)果
[age: 16 weight: 120, age: 11 weight: 54, age: 30 weight: 100]
sort by age: [age: 11 weight: 54, age: 16 weight: 120, age: 30 weight: 100]
sort by weight: [age: 11 weight: 54, age: 30 weight: 100, age: 16 weight: 120]

ThreadPool 拒絕策略

java提供了4種默認(rèn)的拒絕策略，當(dāng)我們需要實現(xiàn)自己的拒絕策略時，實現(xiàn)RejectedExecutionHandler口中的rejectedExecution方法。將線程池的拒絕策略設(shè)置為自己的拒絕策略即可。

/**
 * A handler for tasks that cannot be executed by a {@link ThreadPoolExecutor}.
 *
 * @since 1.5
 * @author Doug Lea
 */
public interface RejectedExecutionHandler {
    void rejectedExecution(Runnable r, ThreadPoolExecutor executor);
}
public class MyRejectedPolicy implements RejectedExecutionHandler {
    @Override    
    void rejectedExecution(Runnable r, ThreadPoolExecutor executor){
        // 具體的拒絕策略
    }
}
// 設(shè)置拒絕策略
executor.setRejectedExecutionHandler(new MyRejectedPolicy());

優(yōu)點

• 算法可以自由切換;
• 避免使用多重條件判斷;
• 擴展性良好

缺點

• 策略類會增多；
• 所有策略類都需要對外暴露