通過使用池的技術，有效減少了大量細粒度的物件的重複生成。

0x00 組織結構

• Flyweight：抽象享元類，聲明瞭可以想外界提供內部狀態的方法，同時還可以設定外部狀態。
• ConcreteFlyweight：具體享元類，通常和單例模式組合使用。
• UnsharedConcreteFlyweight（非共享具體享元類）：並不是所有的抽象享元類的子類都需要被共享，不能被共享的子類可設計為非共享具體享元類；當需要一個非共享具體享元類的物件時可以直接通過例項化建立。
• FlyweightFactory：享元池，享元池一般設計為一個儲存“鍵值對”的集合，可以結合工廠模式進行設計。
  

0x01 示例

以組裝PC為例，其中CPU充當內部狀態，Computer充當外部狀態。

package com.kkk.pattern.flyweight;

/**
 * 充當享元類
 * Created by z3jjlzt on 2018/1/10.
 */
public abstract class CPU {

    //獲取內部狀態
    public abstract String getCPUName();

    //注入外部狀態
    public  void setComputer(Computer computer) {
        System.out.println("把型號為 "
 
 + getCPUName() + "的cpu安裝在型號為 " + computer + "的電腦上");
    }
}

/**
 * 充當具體享元
 * Created by z3jjlzt on 2018/1/10.
 */
public class AMDCPU extends CPU {

    private AMDCPU() {}

    private static class Instance{
        private static final AMDCPU cpu = new AMDCPU();
    }

    public static AMDCPU getInstance
 
() {
        return Instance.cpu;
    }

    @Override
    public String getCPUName() {
        return "AMD";
    }
}

/**
 * 充當具體享元
 * Created by z3jjlzt on 2018/1/10.
 */
public class IntelCPU extends CPU {

    private IntelCPU() {}

    private static class Instance{
        private static final IntelCPU cpu = new IntelCPU();
    }

    public static IntelCPU getInstance() {
        return Instance.cpu;
    }

    @Override
    public String getCPUName() {
        return "Intel";
    }
}

/**
 * 充當外部狀態
 * Created by z3jjlzt on 2018/1/10.
 */
public class Computer {
    private String name;

    public Computer(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        final StringBuffer sb = new StringBuffer("Computer{");
        sb.append("name='").append(name).append('\'');
        sb.append('}');
        return sb.toString();
    }
}

import java.util.HashMap;

/**
 * 充當共享池
 * Created by z3jjlzt on 2018/1/10.
 */
public class CPUFactory {

    private static HashMap<String, CPU> fpool = new HashMap<>();

    private CPUFactory() {
    }

    private static class Instance {
        private static final CPUFactory cf = new CPUFactory();
    }

    public static CPUFactory getInstance() {
        return Instance.cf;
    }

    public static CPU getCPU(String type) {
        if (null == fpool.get(type)) {
            switch (type) {
                case "AMD":
                    fpool.put("AMD", AMDCPU.getInstance());
                    break;
                case "Intel":
                    fpool.put("Intel", IntelCPU.getInstance());
                    break;
                default:
                    break;
            }
        }
        return fpool.get(type);
    }
}

/**
 * Created by z3jjlzt on 2018/1/10.
 */
public class Client {
    public static void main(String[] args) {
        CPUFactory cpuFactory = CPUFactory.getInstance();
        CPU amd1 = cpuFactory.getCPU("AMD");
        CPU amd2 = cpuFactory.getCPU("AMD");
        CPU intel1 = cpuFactory.getCPU("Intel");
        System.out.println(amd1 == amd2);
        System.out.println(amd1 == intel1);
        amd1.setComputer(new Computer("新華同方"));
        intel1.setComputer(new Computer("華碩"));
    }
}
結果：
package com.kkk.pattern.flyweight;

/**
 * 充當享元類
 * Created by z3jjlzt on 2018/1/10.
 */
public abstract class CPU {

    //獲取內部狀態
    public abstract String getCPUName();

    //注入外部狀態
    public  void setComputer(Computer computer) {
        System.out.println("把型號為 " + getCPUName() + "的cpu安裝在型號為 " + computer + "的電腦上");
    }
}

/**
 * 充當具體享元
 * Created by z3jjlzt on 2018/1/10.
 */
public class AMDCPU extends CPU {

    private AMDCPU() {}

    private static class Instance{
        private static final AMDCPU cpu = new AMDCPU();
    }

    public static AMDCPU getInstance() {
        return Instance.cpu;
    }

    @Override
    public String getCPUName() {
        return "AMD";
    }
}

/**
 * 充當具體享元
 * Created by z3jjlzt on 2018/1/10.
 */
public class IntelCPU extends CPU {

    private IntelCPU() {}

    private static class Instance{
        private static final IntelCPU cpu = new IntelCPU();
    }

    public static IntelCPU getInstance() {
        return Instance.cpu;
    }

    @Override
    public String getCPUName() {
        return "Intel";
    }
}

/**
 * 充當外部狀態
 * Created by z3jjlzt on 2018/1/10.
 */
public class Computer {
    private String name;

    public Computer(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        final StringBuffer sb = new StringBuffer("Computer{");
        sb.append("name='").append(name).append('\'');
        sb.append('}');
        return sb.toString();
    }
}

import java.util.HashMap;

/**
 * 充當共享池
 * Created by z3jjlzt on 2018/1/10.
 */
public class CPUFactory {

    private static HashMap<String, CPU> fpool = new HashMap<>();

    private CPUFactory() {
    }

    private static class Instance {
        private static final CPUFactory cf = new CPUFactory();
    }

    public static CPUFactory getInstance() {
        return Instance.cf;
    }

    public static CPU getCPU(String type) {
        if (null == fpool.get(type)) {
            switch (type) {
                case "AMD":
                    fpool.put("AMD", AMDCPU.getInstance());
                    break;
                case "Intel":
                    fpool.put("Intel", IntelCPU.getInstance());
                    break;
                default:
                    break;
            }
        }
        return fpool.get(type);
    }
}

/**
 * Created by z3jjlzt on 2018/1/10.
 */
public class Client {
    public static void main(String[] args) {
        CPUFactory cpuFactory = CPUFactory.getInstance();
        CPU amd1 = cpuFactory.getCPU("AMD");
        CPU amd2 = cpuFactory.getCPU("AMD");
        CPU intel1 = cpuFactory.getCPU("Intel");
        System.out.println(amd1 == amd2);
        System.out.println(amd1 == intel1);
        amd1.setComputer(new Computer("新華同方"));
        intel1.setComputer(new Computer("華碩"));
    }
}
結果:
true
false
把型號為 AMD的cpu安裝在型號為 Computer{name='新華同方'}的電腦上
把型號為 Intel的cpu安裝在型號為 Computer{name='華碩'}的電腦上

0xff 總結

• 優點：大大減少相同物件的數量，內外部狀態互相獨立。
• 缺點：需要區分內外部狀態，增加系統設計難度。
• 適用場景：一個系統有大量相同或者相似的物件，造成記憶體的大量耗費。