前言
当一个类的属性足够多时，且不确定在构造对象时，哪些属性必须，哪些是非必须的。此时，创建该对象这件事情变的不确定。我们也许要排列组合般的写上无数个构造函数，这样显得十分臃肿。此时，一个较为优雅的设计模式应运而生—建造者模式（Builder Pattern）
传统形式
传统的建造者模式由四大部分组成：Product（最终要生成的对象）、Builder（创建着的抽象基类）、ConcreteBuilder（Builder实现类，负责干活）、Director（调用类）
代码示例：
下面是一个Product电脑类，由必须部分和非必须部分组成
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| @Data
public class Computer {
private String cpu;
private String gpu;
private String os;
private String keyboard;
private String display;
public Computer(String cpu, String gpu) {
this.cpu = cpu;
this.gpu = gpu;
}
}
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定义一个抽象的Builder：
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| public interface Builder<T> {
T build();
}
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定义两个不通品牌电脑的实现：
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| public class WindowsComputerBuilder implements Builder<Computer> {
private Computer computer;
public WindowsComputerBuilder builder(String cpu, String gpu) {
this.computer = new Computer(cpu, gpu);
return this;
}
public WindowsComputerBuilder os(String os) {
computer.setOs(os);
return this;
}
public WindowsComputerBuilder keyboard(String keyboard) {
computer.setKeyboard(keyboard);
return this;
}
public WindowsComputerBuilder display(String display) {
computer.setDisplay(display);
return this;
}
@Override
public Computer build() {
return computer;
}
}
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| public class MacComputerBuilder implements Builder<Computer> {
private Computer computer;
public MacComputerBuilder builder(String cpu, String gpu) {
this.computer = new Computer(cpu, gpu);
return this;
}
public MacComputerBuilder os(String os) {
computer.setOs(os);
return this;
}
public MacComputerBuilder keyboard(String keyboard) {
computer.setKeyboard(keyboard);
return this;
}
public MacComputerBuilder display(String display) {
computer.setDisplay(display);
return this;
}
@Override
public Computer build() {
return computer;
}
}
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Director生产两种不同的电脑：
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| public class Director {
public static void main(String[] args) {
Computer windows = new WindowsComputerBuilder().builder("intel", "rtx")
.os("windows")
.keyboard("windows键盘")
.display("windows显示器").build();
System.out.println(windows);
Computer macbook = new MacComputerBuilder().builder("m2", "intel hd")
.os("macos")
.keyboard("mac键盘")
.display("mac显示器").build();
System.out.println(macbook);
}
}
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可以看到，Builder Pattern生成对象时一般都会执行链式调用，且区分除了必须和非必须的参数
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| Computer(cpu=intel, gpu=rtx, os=windows, keyboard=键盘, display=显示器)
Computer(cpu=m2, gpu=intel hd, os=macos, keyboard=键盘, display=显示器)
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这样做，构建对象时显得更为优雅
JAVA中简化方式
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| @Data
public class Computer {
private String cpu;
private String gpu;
private String os;
private String keyboard;
private String display;
public Computer(Builder builder) {
this.cpu = builder.cpu;
this.gpu = builder.gpu;
this.os = builder.os;
this.keyboard = builder.keyboard;
this.display = builder.display;
}
public static class Builder{
private String cpu;
private String gpu;
private String os;
private String keyboard;
private String display;
public Builder(String cpu, String gpu) {
this.cpu = cpu;
this.gpu = gpu;
}
public Builder os(String os) {
this.os = os;
return this;
}
public Builder keyboard(String keyboard) {
this.keyboard = keyboard;
return this;
}
public Builder display(String display) {
this.display = display;
return this;
}
public Computer build() {
return new Computer(this);
}
}
}
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将构建者成为了该类的一个静态内部类
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| public class Director {
public static void main(String[] args) {
Computer windows = new Computer.Builder("intel", "rtx")
.os("windows")
.keyboard("windows键盘")
.display("windows显示器").build();
System.out.println(windows);
Computer macbook = new Computer.Builder("m2", "intel hd")
.os("macos")
.keyboard("mac键盘")
.display("mac显示器").build();
System.out.println(macbook);
}
}
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调用结果：
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| Computer(cpu=intel, gpu=rtx, os=windows, keyboard=windows键盘, display=windows显示器)
Computer(cpu=m2, gpu=intel hd, os=macos, keyboard=mac键盘, display=mac显示器)
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使用工具
lombok该SDK提供了@Builder注解，可以一键构建对象