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WHAT 什么是建造者

建造者模式（builder）鸳谜，將一個復雜對象的構建與它的表示分離，使得同樣的構建過程可以創(chuàng)建不同的表示式廷。建造者主要用于創(chuàng)建一些復雜的對象咐扭，這些對象內(nèi)部的構建通常是非常復雜的過程。

建造者UML

WHY 為什么使用建造者

• 如果一個類具有非常多屬性滑废，為了避免構造函數(shù)的參數(shù)列表太長草描，影響可讀性。
• 建造者的好處是對象的創(chuàng)建代碼和對象本身的代碼解耦策严，調(diào)用者無需關心復雜的構建過程穗慕。提高代碼的易用性
• 建造者能讓用戶使用到的是一個構造完整的對象，用戶不會調(diào)用到一個未構造完整的對象妻导。

HOW 建造者的實現(xiàn)(JAVA)

場景：創(chuàng)建一個資源配置對象逛绵，內(nèi)部可能存在復雜的構造過程

• 資源配置類和建造器

package builderDesignPattern;

public class ResourceConfig {
    private String url;
    private String pwd;
    private String user;
    private int coreSize;
    private int maxSize;
    
    private ResourceConfig() {
    }

    public ResourceConfig(String url, String user, String pwd, int coreSize, int maxSize) {
        this.url = url;
        this.user = user;
        this.pwd = pwd;
        this.coreSize = coreSize;
        this.maxSize = maxSize;
    }


    @Override
    public String toString() {
        return "ResourceConfig{" +
                "url='" + url + '\'' +
                ", pwd='" + pwd + '\'' +
                ", user='" + user + '\'' +
                ", coreSize=" + coreSize +
                ", maxSize=" + maxSize +
                '}';
    }

    public static class ResourceConfigBuilder {
        private String url;
        private String pwd;
        private String user;
        private int coreSize;
        private int maxSize;

        public ResourceConfigBuilder setUrl(String url) {
            this.url = url;
            return this;
        }

        public ResourceConfigBuilder setPwd(String pwd) {
            this.pwd = pwd;
            return this;
        }

        public ResourceConfigBuilder setUser(String user) {
            this.user = user;
            return this;
        }

        public ResourceConfigBuilder setCoreSize(int coreSize) {
            this.coreSize = coreSize;
            return this;
        }


        public ResourceConfigBuilder setMaxSize(int maxSize) {
            this.maxSize = maxSize;
            return this;
        }

        public ResourceConfig build() {
            if (isEmpty(url)) {
                throw new IllegalArgumentException();
            }

            if (isEmpty(user)) {
                throw new IllegalArgumentException();
            }

            if (isEmpty(pwd)) {
                throw new IllegalArgumentException();
            }

            if (coreSize > maxSize || coreSize <= Integer.MIN_VALUE) {
                throw new IllegalArgumentException();
            }

            if (coreSize > maxSize || maxSize >= Integer.MAX_VALUE) {
                throw new IllegalArgumentException();
            }

            return new ResourceConfig(url, user, pwd, coreSize, maxSize);
        }

        private boolean isEmpty(String s) {
            return s == null || s.isEmpty();
        }

    }
}

• 客戶端

public class BuilderClient {
    public static void main(String[] args) {
        String url = "https://www.xxoo.com";
        String user = "admin";
        String pwd = "a123";
        ResourceConfig resourceConfig = new ResourceConfig.ResourceConfigBuilder().setCoreSize(1)
                .setMaxSize(5).setUrl(url).setUser(user).setPwd(pwd).build();
        System.out.println(resourceConfig.toString());
    }
}

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