builder设计模式学习笔记

在面向对象编程过程中，通常通过构造器new的方式来获取一个类的对象。这种方式比较适合对象属性较少的情形。当对象的属性过多时，这种方式就不太适合了。以一个食物商品为例，它拥有总脂肪量，胆固醇，钠，蔗糖等20多个属性，当我们生成该商品的对象时，有些属性并不需要，于是我们会以重叠构造器的方式来编写这个类。

public class Food01 {

private final int servingSize;

private final int serving;

private int calories = 0;

private int fat = 0;

private int sodium = 0;

private int carbohydrate = 0;

public Food01(int servingSize){
this(servingSize,0);
}

public Food01(int servingSize,int serving){
this(servingSize,serving,0);
}

public Food01(int servingSize,int serving,int calories){
this(servingSize,serving,calories,0);
}

public Food01(int servingSize,int serving,int calories,int fat){
this(servingSize,serving,calories,fat,0);
}

public Food01(int servingSize,int serving,int calories,int fat,int sodium){
this(servingSize,serving,calories,fat,sodium,0);
}

public Food01(int servingSize,int serving,int calories,int fat,int sodium,int carbohydrate){
this.servingSize = servingSize;
this.serving = serving;
this.calories = calories;
this.fat = fat;
this.sodium = sodium;
this.carbohydrate = carbohydrate;
}
}

代码中只列举了六个属性，当属性20多个时，以这种方式生成对象，将会导致代码庞大且难以编写和阅读。对于这种情况还有一种方式可以替代，就是通过javaBeans模式，通过set和get方法来设置对象的属性，避免了大量的构造器。

Food food = new Food();
food.setServingSize(servingSize);
food.setServing(serving);
food.setcalories(calories);

javaBeans模式虽然解决了重叠构造器所遇到的问题，但是它有一个严重的缺点。它将所需属性的对象的生成分成了好几步完成，在food对象set属性时可能会遭遇多线程问题，这需要程序员编写额外代码来保证线程安全。采用Builder模式就可以解决前面两种方式遇到的问题。

Builder模式是在采用静态内部类Builder，通过该静态内部类调用类似setter的方法来设置属性，在通过build方法生成所需的对象。具体代码如下：

public class Food {
private final int servingSize;
private final int serving;
private final int calories;
private final int fat;
private final int sodium;
private final int carbohydrate;

private Food(Builder builder) {
serving = builder.serving;
servingSize = builder.servingSize;
calories = builder.calories;
fat = builder.fat;
sodium = builder.sodium;
carbohydrate = builder.carbohydrate;

}

public int getCalories() {
return calories;
}

static class Builder{
private final int servingSize;
private final int serving;

private int calories = 0;
private int fat = 0;
private int sodium = 0;
private int carbohydrate = 0;

public Builder(int servingSize,int serving) {
this.servingSize = servingSize;
this.serving = serving;
}

public Builder calories(int calories) {
this.calories = calories;
return this;
}

public Builder fat(int fat) {
this.fat = fat;
return this;
}

public Builder sodium(int sodium) {
this.sodium = sodium;
return this;
}

public Builder carbohydrate(int carbohydrate) {
this.carbohydrate = carbohydrate;
return this;
}

public Food build() {
return new Food(this);
}
}

}

builder的每一个setter方法都返回builder本身，用户可以调用不同的setter方法来设置所需的属性，最后将builder对象通过构造器的方式注入到Food对象，并将属性复制到Food对象。采用builder模式，代码就相对容易编写，且易于阅读。