定义一组算法，并封装好其中每一个，使它们可以互相替换。策略模式使算法的变化独立于使用它的客户。

一个 redis 中的例子：redis 内存写满之后会通过不同的策略将数据删除，继续提供服务。使用策略模式模拟redis中的4中淘汰策略（random、ttl、lru、lfu）

public interface EvictionStrategy {

    /**
     * 筛选出淘汰的 key
     */
    List<String> selectKeys();
}

public class LfuStrategy implements EvictionStrategy {

    @Override
    public List<String> selectKeys() {
        System.out.println("使用 lfu 淘汰策略");
        return Collections.singletonList("lfu_key");
    }
}

public class LruStrategy implements EvictionStrategy {

    @Override
    public List<String> selectKeys() {
        System.out.println("使用 lru 淘汰策略");
        return Collections.singletonList("lru_key");
    }
}

public class RandomStrategy implements EvictionStrategy {

    @Override
    public List<String> selectKeys() {
        System.out.println("使用 random 淘汰策略");
        return Collections.singletonList("random_key");
    }
}

public class TtlStrategy implements EvictionStrategy {

    @Override
    public List<String> selectKeys() {
        System.out.println("使用 ttl 淘汰策略");
        return Collections.singletonList("ttl_key");
    }
}

public class EvictionStrategyFactory {

    private static final Map<String, EvictionStrategy> STRATEGY_MAP = new HashMap<>();

    static {
        STRATEGY_MAP.put("ttl", new TtlStrategy());
        STRATEGY_MAP.put("lru", new LruStrategy());
        STRATEGY_MAP.put("lfu", new LfuStrategy());
        STRATEGY_MAP.put("random", new RandomStrategy());
    }

    public static EvictionStrategy getStrategy(String type) {
        return STRATEGY_MAP.get(type);
    }

}

public class RedisServer {

    private EvictionStrategy strategy;

    public RedisServer(final EvictionStrategy strategy) {
        this.strategy = strategy;
    }

    // 根据策略淘汰缓存
    public void clean() {
        List<String> keys = strategy.selectKeys();
        System.out.println("淘汰缓存: " + keys);
    }

    // 动态修改缓存淘汰策略
    public void changeStrategy(String type) {
        System.out.println("修改缓存策略: " + type);
        strategy = EvictionStrategyFactory.getStrategy(type);
    }
}

测试类

public class StrategyApp {

    public static void main(String[] args) {
        String type = getTypeFromConfig("redis.conf");
        EvictionStrategy strategy = EvictionStrategyFactory.getStrategy(type);
        RedisServer server = new RedisServer(strategy);
        server.clean();

        server.changeStrategy("lfu");
        server.clean();

    }

    private static String getTypeFromConfig(String configFilePath) {
        return "lru";
    }
}