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我们在开发时经常会遇到一堆的if else …, 或者switch, 比如我们常见的全局异常处理等, 像类似这种很多if else 或者多场景模式下, 策略模式是非常受欢迎的一种设计模式, 然而, 一个好的策略模式却不是那么容易写出来.

我在工作中也因为写烦了switch,if else 觉得很不优雅, 因此,我考虑是否有一套统一的解决方案呢?

思考我问题的初衷, 在什么策略下, 满足什么条件执行什么动作, 返回什么值, 这就是策略模式需要解决的核心问题, 大眼一看好似有点类似状态机? 然而它并不是状态机, 状态机是比较笨重, 而策略机应该是足够轻量的.

我们再来看核心问题,关于什么策略,满足什么条件执行什么动作,返回什么值, 是一个明显的dsl语法, 因此, 我的基本语法糖已确立: strategy.of().when().perform()或者strategy.of().perform(), 因为有时我们并不需要条件, 仅仅策略即可.

我们要实现上述语法糖, 就得设计一套规则, 使其可以满足dsl, 并且是合理的, 如此, 基本定义已确定, 如下:

/*** {@link StrategyMachineBuilder}* @param <S> strategy* @param <C> context* @param <R> result*/
public interface StrategyMachineBuilder<S,C,R> {Of<S,C,R> of(S s);StrategyMachine<S,C,R> build(String id);
}

/*** {@link Of}* @param <S> strategy* @param <C> context* @param <R> result*/
public interface Of<S,C,R> {When<S,C,R> when(Condition<S,C,R> condition);StrategyMachineBuilder<S,C,R> perform(Action<C,R> action);
}

/*** {@link When}* @param <S> strategy* @param <C> context* @param <R> result*/
public interface When<S,C,R> {StrategyMachineBuilder<S,C,R> perform(Action<C,R> action);
}

/*** {@link Condition}* @param <S> strategy* @param <C> context* @param <R> result*/
public interface Condition<S,C,R> {boolean isSatisfied(S s,C c);
}

/*** {@link Action}* @param <C> context* @param <R> result*/
public interface Action<C,R> {R apply(C c);
}

/*** {@link Strategy}* @param <S> strategy* @param <C> context* @param <R> result*/
public interface Strategy<S, C, R> {S strategy();Condition<S,C,R> condition();Action<C, R> action();Strategy<S,C,R> strategy(S s);Strategy<S,C,R> condition(Condition<S,C,R> condition);Strategy<S,C,R> action(Action<C,R> action);
}

/*** {@link StrategyMachine}* @param <S> strategy* @param <C> context* @param <R> result*/
public interface StrategyMachine<S,C,R> {R apply(S s, C c);
}

如此: 架构已经构建完毕, 剩下的工作就很简单了, 实现此架构即可.

/*** {@link StrategyMachineBuilderImpl}* @param <S> strategy* @param <C> context* @param <R> result*/
class StrategyMachineBuilderImpl<S,C,R> implements StrategyMachineBuilder<S,C,R>{private final Map<S, List<Strategy<S,C,R>>> map = new ConcurrentHashMap<>();@Overridepublic Of<S, C, R> of(S s) {map.computeIfAbsent(s, k -> new ArrayList<>());Strategy<S,C,R> strategy = new StrategyImpl();map.get(s).add(strategy);return new OfImpl(strategy);}@Overridepublic StrategyMachine<S, C, R> build(String id) {StrategyMachineImpl<S, C, R> machine = new StrategyMachineImpl<>(map);StrategyCache.put(id, machine);return machine;}public class OfImpl implements Of<S,C,R>{private final Strategy<S,C,R> strategy;OfImpl(Strategy<S,C,R> strategy){this.strategy = strategy;}@Overridepublic When<S, C, R> when(Condition<S,C,R> condition) {this.strategy.condition(condition);return new WhenImpl(strategy);}@Overridepublic StrategyMachineBuilder<S, C, R> perform(Action<C, R> action) {this.strategy.action(action);return StrategyMachineBuilderImpl.this;}}public class WhenImpl implements When<S,C,R> {private final Strategy<S,C,R> strategy;WhenImpl(Strategy<S,C,R> strategy){this.strategy = strategy;}@Overridepublic StrategyMachineBuilder<S, C, R> perform(Action<C, R> action) {this.strategy.action(action);return StrategyMachineBuilderImpl.this;}}public class StrategyImpl implements Strategy<S, C, R> {private S strategy;private Condition<S,C,R> condition;private Action<C, R> action;@Overridepublic S strategy() {return this.strategy;}@Overridepublic Condition<S,C,R> condition() {return this.condition;}@Overridepublic Action<C, R> action() {return this.action;}@Overridepublic Strategy<S, C, R> strategy(S s) {this.strategy = s;return this;}@Overridepublic Strategy<S, C, R> condition(Condition<S,C,R> condition) {this.condition = condition;return this;}@Overridepublic Strategy<S, C, R> action(Action<C, R> action) {this.action = action;return this;}}
}

/*** Strategy Machine Impl* @param <S> strategy* @param <C> context* @param <R> result*/
class StrategyMachineImpl<S,C,R> implements StrategyMachine<S,C,R> {private final Map<S, List<Strategy<S,C,R>>> map;public StrategyMachineImpl(Map<S, List<Strategy<S,C,R>>> map){this.map = map;}@Overridepublic R apply(S s, C c) {List<Strategy<S, C, R>> strategies = map.get(s);if (strategies==null||strategies.isEmpty()){throw new RuntimeException("no strategy found for "+s);}for (Strategy<S, C, R> strategy : strategies) {// 如果没有condition，直接执行actionif (strategy.condition()==null) {return strategy.action().apply(c);}// 如果有condition，先判断是否满足condition，满足则执行actionif (strategy.condition().isSatisfied(s,c)){return strategy.action().apply(c);}}// 未发现策略关于s的conditionthrow new RuntimeException("no strategy found of met condition for "+s);}
}

/*** Strategy Machine Factory*/
public class StrategyMachineFactory {public static <S,C,R> StrategyMachineBuilder<S,C,R> create() {return new StrategyMachineBuilderImpl<>();}public static <S,C,R> StrategyMachine<S,C,R> get(String id) {return (StrategyMachine<S, C, R>) StrategyCache.get(id);}
}

/*** {@link StrategyCache}*/
class StrategyCache {private static final Map<String,StrategyMachine<?,?,?>> CACHE = new java.util.concurrent.ConcurrentHashMap<>();public static void put(String id, StrategyMachine<?,?,?> machine) {CACHE.put(id, machine);}public static StrategyMachine<?,?,?> get(String id) {return CACHE.get(id);}
}

如此, 策略机已实现完毕. 下面给出两种场景例子
一. 不同年龄吃不同分量的药
Example:
Under the age of 12, take 20 milligrams of medication per day;
12-18 years old, taking 30 milligrams a day
18-30 years old, taking 40 milligrams a day
30-50 years old, taking 45 milligrams a day
Eating 42 milligrams for those over 50 years old

class MedicineStrategy {private static StrategyMachine<String, MedicineContext, Void> strategy;static {StrategyMachineBuilder<String, MedicineContext, Void> machineBuilder = StrategyMachineFactory.create();strategy = machineBuilder.of("").when((s, c) -> c.age < 12).perform((c) -> {System.out.println("Under the age of 12, take 20 milligrams of medication per day;");return Void.TYPE.cast(null);}).of("").when((s, c) -> c.age >= 12 && c.age < 18).perform((c) -> {System.out.println("12-18 years old, taking 30 milligrams a day");return Void.TYPE.cast(null);}).of("").when((s, c) -> c.age >= 18 && c.age < 30).perform((c) -> {System.out.println("18-30 years old, taking 40 milligrams a day");return Void.TYPE.cast(null);}).of("").when((s, c) -> c.age >= 30 && c.age < 50).perform((c) -> {System.out.println("30-50 years old, taking 45 milligrams a day");return Void.TYPE.cast(null);}).of("").when((s, c) -> c.age >= 50).perform((c) -> {System.out.println("Eating 42 milligrams for those over 50 years old");return Void.TYPE.cast(null);}).build("medicine");}public static StrategyMachine<String, MedicineContext, Void> get() {// StrategyMachine<String, MedicineContext, Void> strategy = StrategyMachineFactory.get("medicine");return strategy;}@Data@AllArgsConstructor@NoArgsConstructorpublic static class MedicineContext {private int age;}public static void main(String[] args) {get().apply("", new MedicineContext(10));}}

二. 计算机

		StrategyMachineBuilder<String, StrategyContext, Number> machineBuilder = StrategyMachineFactory.create();machineBuilder.of("加法").perform(strategyContext -> strategyContext.a + strategyContext.b);machineBuilder.of("减法").perform(strategyContext -> strategyContext.a - strategyContext.b);machineBuilder.of("乘法").perform(strategyContext -> strategyContext.a * strategyContext.b);// 除法,当c==1时,忽略小数位, 当c==2时不忽略machineBuilder.of("除法").when((s, strategyContext) -> strategyContext.c == 1).perform(strategyContext -> strategyContext.a / strategyContext.b);machineBuilder.of("除法").when((s, strategyContext) -> strategyContext.c == 2).perform(strategyContext -> (strategyContext.a * 1.0d) / (strategyContext.b * 1.0d));StrategyMachine<String, StrategyContext, Number> strategyMachine = machineBuilder.build("test");// StrategyMachine<String, StrategyContext, Number> strategyMachine =  StrategyMachineFactory.get("test");System.out.println(strategyMachine.apply("加法", new StrategyContext(1, 2, 1)));System.out.println(strategyMachine.apply("减法", new StrategyContext(1, 2, 1)));System.out.println(strategyMachine.apply("乘法", new StrategyContext(1, 2, 1)));System.out.println(strategyMachine.apply("除法", new StrategyContext(1, 2, 1)));System.out.println(strategyMachine.apply("除法", new StrategyContext(1, 2, 2)));

源码地址: https://github.com/zhangpan-soft/dv-commons