java的Builder原理和实现详解

首先给一个简单的Builder设计模式的例子：

主实现类代码如下：

/**
 * 实体类 包含一个静态内部类 Builder
 */
public class CompanyClient {
    public String companyName;
    // 用final修饰的成员变量表示常量，只能被赋值一次，赋值后值无法改变！ final修饰的变量有三种：静态变量、实例变量和局部变量。
    public String companyAddress;

    public double companyRegfunds;
    public String mPerson;
    public String mType;
     //构造方法
     public CompanyClient() {
         this(new Builder());
     }
     //构造方法
     public CompanyClient(Builder builder){
         this.companyName = builder.companyName;
         this.companyAddress = builder.companyAddress;
         this.companyRegfunds = builder.companyRegfunds;
         this.mPerson = builder.person;
         this.mType = builder.type;
     }
     public String getCompanyName() {
         return companyName;
     }

     public String getCompanyAddress() {
         return companyAddress;
     }

     public double getCompanyRegfunds() {
         return companyRegfunds;
     }

     public String getmPerson() {
         return mPerson;
     }

     public String getmType() {
         return mType;
     }

     public Builder newBuilder() {
         return new Builder(this);
     }

     @Override //重写toString方法后，当print这个对象的时候，会默认的调用toString()这个方法。
     public String toString() {
         return "CompanyClient{" +
                 "companyName='" + companyName + '\'' +
                 ", companyAddress='" + companyAddress + '\'' +
                 ", companyRegfunds=" + companyRegfunds +"千万"+
                 ", mPerson=" + mPerson +
                 ", mType='" + mType + '\'' +
                 '}';
     }
     /**
     *静态内部类 Builder
     */
     public static class Builder{
        public String companyName;
        public String companyAddress;
        public  double companyRegfunds;
        public  String person;
        public String type;
        //构造方法
        public Builder() {
            companyName = companyName;
            companyAddress = companyAddress;
            companyRegfunds = companyRegfunds;
            person = person;
            type = type;

        }
        //构造方法
        Builder(CompanyClient companyClient){
            this.companyName = companyClient.companyName;
            this.companyAddress = companyClient.companyAddress;
            this.companyRegfunds = companyClient.companyRegfunds;
            this.person = companyClient.mPerson;
            this.type = companyClient.mType;
        }

        public Builder setCompanyName(String name) {
            companyName = name;
            return this;
        }

        public Builder setCompanyAddress(String address) {
            companyAddress = address;
            return this;
        }

        public Builder setCompanyRegfunds(double regfunds) {
            companyRegfunds = regfunds;
            return this;
        }

        public Builder setmPerson(String per) {
            person = per;
            return this;
        }

        public Builder setmType(String typeStr) {
            type = typeStr;
            return this;
        }
         //构建一个实体
         public CompanyClient build() {
             return new CompanyClient(this);
         }
    }
 }

测试类实现代码如下：

public class TestBuilder {
    public static void main(String[] args) {

        CompanyClient client = new CompanyClient.Builder()
                .setCompanyName("alibaba")
                .setCompanyAddress("wangjing")
                .setCompanyRegfunds(5)
                .setmPerson("10000")
                .build();
        System.out.println(client);
        System.out.println("---------------------");
        CompanyClient.Builder builder = new CompanyClient.Builder();
        builder.setCompanyName("huawei");
        builder.setCompanyAddress("haidian");
        builder.setCompanyRegfunds(20);
        builder.setmType("communication");
        CompanyClient client1 = builder.build();
        System.out.println(client1);
        System.out.println("---------------------");
        CompanyClient.Builder build1 = client1.newBuilder();
        build1.setCompanyName("baidu");
        CompanyClient client2 = build1.build();
        System.out.println(client2);
    }
}

输出结果如下：

CompanyClient{companyName='alibaba', companyAddress='wangjing', companyRegfunds=5.0千万, mPerson=10000, mType='null'}
---------------------
CompanyClient{companyName='huawei', companyAddress='haidian', companyRegfunds=20.0千万, mPerson=null, mType='communication'}
---------------------
CompanyClient{companyName='baidu', companyAddress='haidian', companyRegfunds=20.0千万, mPerson=null, mType='communication'}

首先看main函数中的第一行代码：

CompanyClient client = new CompanyClient.Builder()
                .setCompanyName("alibaba")
                .setCompanyAddress("wangjing")
                .setCompanyRegfunds(5)
                .setmPerson("10000")
                .build();

之所以可以这么执行，是因为内部静态类Builder里定义了一份与CompanyClient类一模一样的变量，通过一系列的成员函数进行设置属性值，但是返回值都是this，也就是都是Builder对象，最后提供了一个build函数用于创建CompanyClient对象，返回的是CompanyClient对象，对应的构造函数在CompanyClient 类中进行定义，也就是构造函数的入参是Builder对象，然后依次对自己的成员变量进行赋值，对应的值都是Builder对象中的值。此外Builder类中的成员函数返回Builder对象自身的另一个作用就是让它支持链式调用，使代码可读性大大增强。

总结下，Java环境下builder设计模式：

定义一个静态内部类Builder，内部的成员变量和外部类一样

Builder类通过一系列的方法用于成员变量的赋值，并返回当前对象本身（this）

Builder类提供一个build方法或者create方法用于创建对应的外部类，该方法内部调用了外部类的一个私有构造函数，该构造函数的参数就是内部类Builder

外部类提供一个私有构造函数供内部类调用，在该构造函数中完成成员变量的赋值，取值为Builder对象中对应的值

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