十一、享元Flyweight（结构型）

享元模式可以帮助你运用共享技术有效的支持大量细粒度的对象。Flyweight是一个共享对象，它可以同时在多个场景中使用，并且在每个场景中flyweight都可以作为一个独立的对象——这点和非共享对象的实例没有区别。例如，在文本编辑器中，所有的字符都会在开始时初始化，并在全局内共享所有字符，而不是每个字符就创建对象。

Flyweight适用于有大量的对象，且这些对象可以分为外部状态和内部状态两个部分，这样就可以将内部状态分离开来，创建独立的共享对象。在使用Flyweight时，一般会使用一个FlyweightFactory来帮助用户创建和查找某个特定对象。

public interface Flyweight {public String operation();public void setLeft(Flyweight node);public void setRight(Flyweight node);public Flyweight getLeft();public Flyweight getRight();}public class ConcreteFlyweight implements Flyweight{private Flyweight intrinsicState = null;private Flyweight left = null;private Flyweight right = null;public ConcreteFlyweight(String value){intrinsicState = FlyweightFactory.getFlyweight(value);}@Overridepublic String operation() {StringBuilder ss = new StringBuilder(intrinsicState.operation());if(left != null) ss.append(left.operation());if(right != null) ss.append(right.operation());return ss.toString();}@Overridepublic void setLeft(Flyweight node) {left = node;}@Overridepublic void setRight(Flyweight node) {right = node;}@Overridepublic Flyweight getLeft() {return left;}@Overridepublic Flyweight getRight() {return right;}}public class FlyweightFactory {private static Map<String, Flyweight> pool = new HashMap<>();public static Flyweight getFlyweight(String key){if(pool.containsKey(key)){return pool.get(key);} else {Flyweight fw = new internalFlyweight(key);pool.put(key, fw);return fw;}}private static class internalFlyweight implements Flyweight{private String value = null;public internalFlyweight(String value){this.value = value;}@Overridepublic String operation() {return value;}@Overridepublic void setLeft(Flyweight node) {throw new RuntimeException("unsupport!");}@Overridepublic void setRight(Flyweight node) {throw new RuntimeException("unsupport!");}@Overridepublic Flyweight getLeft() {throw new RuntimeException("unsupport!");}@Overridepublic Flyweight getRight() {throw new RuntimeException("unsupport!");}}}public class Client {@Testpublic void test(){Flyweight root = buildTree(new String[]{"a", "b", "c","a","b","c"});System.out.println(root.operation());}//构建一棵节点树，这棵树按照广度优先，一层一层向下public Flyweight buildTree(String strs[]){return dfs(strs, 0);}private Flyweight dfs(String strs[], int index){Flyweight root = new ConcreteFlyweight(null);if(index < strs.length){root = new ConcreteFlyweight(strs[index]);}if(2*index+1 < strs.length){root.setLeft(dfs(strs, 2*index+1));}if(2*index+2 < strs.length){root.setRight(dfs(strs, 2*index+2));}return root;}}

这里将字符作为Flyweight对象，FlyweightFactory内部包含了一个所有Flyweight对象的pool，当查找Flyweight对象时，如果存在pool中，则不再创建而是直接复用，而不存在时，则需要创建。这样字符的内容作为内部状态被共享，而字符间的关系作为外部状态被额外存储在节点信息中。通过Flyweight模式可以在需要大量字符时减少存储字符所需要的空间。