在创建一个对象的时候，构造器参数有点多，而且有些参数还是可选的，再者还有不少同类型的，那就更应该使用 builder 模式了。
使用 Builder 模式的初衷是 把易变性（mutability）移动到Builder类，而使得要被创建的对象变得不可变（immutable）。
传统方式
创建静态内部类 Builder ，此 Builder 和外部类拥有一样的属性。包括一些返回 this对象的 setter方法，和一个 build 方法调用 外部类的 以builder对象为参数构造器 来创建 外部类对象。
@Getter
public class Blog {private final String title;private final String content;private final String label;private final String author;public Blog(BlogBuilder builder){this.title = builder.title;this.content = builder.content;this.label = builder.label;this.author = builder.author;}static class BlogBuilder{private String title;private String content;private String label;private String author;BlogBuilder title(String title){this.title = title;return this;}BlogBuilder content(String content){this.content = content;return this;}BlogBuilder label(String label){this.label = label;return this;}BlogBuilder author(String author){this.author = author;return this;}Blog build(){return new Blog(this);}}
}对象创建过程如下：
@Testpublic void test(){Blog blog = new Blog.BlogBuilder().author("dachuili").content("classic/generic/lombok builder pattern").label("design patterns").title("builder pattern").build();assertSame(blog.getAuthor(), "dachuili");assertSame(blog.getTitle(), "builder pattern");assertSame(blog.getContent(), "classic/generic/lombok builder pattern");assertSame(blog.getLabel(), "design patterns");}Lombok方式
Lombok提供了注解 @Builder ，一步实现 Builder 模式。不要忘了和@Value 一起用。
import lombok.Builder;
import lombok.Data;
import lombok.Value;@Builder
@Value
@Data
public class Article {private String title;private String content;private String label;private String author;
}创建过程：
@Testpublic void test(){Article article = Article.builder().author("dachuili").content("classic/generic/lombok builder pattern").label("design patterns").title("builder pattern").build();assertSame(article.getAuthor(), "dachuili");assertSame(article.getTitle(), "builder pattern");assertSame(article.getContent(), "classic/generic/lombok builder pattern");assertSame(article.getLabel(), "design patterns");}是不是很 easy！很 convenient！
@Builder 帮我们创建了 Builder 对象以及 这些返回 this对象的 setter方法。以下是lombok生成的ArticleBuilder对象 代码。
和传统方式并为区别。
@Generatedpublic static ArticleBuilder builder() {return new ArticleBuilder();}@Generatedpublic static class ArticleBuilder {@Generatedprivate String title;@Generatedprivate String content;@Generatedprivate String label;@Generatedprivate String author;@GeneratedArticleBuilder() {}@Generatedpublic ArticleBuilder title(String title) {this.title = title;return this;}@Generatedpublic ArticleBuilder content(String content) {this.content = content;return this;}@Generatedpublic ArticleBuilder label(String label) {this.label = label;return this;}@Generatedpublic ArticleBuilder author(String author) {this.author = author;return this;}@Generatedpublic Article build() {return new Article(this.title, this.content, this.label, this.author);}}
通用的Builder
借助于 Java 8 提供的 Supplier和 BiConsumer 创建一个通用工具类，好玩是好玩，强行捏了一个builder出来，感觉违背了 builder模式的初衷，回到了 JavaBeans Pattern 那种setter方法。
public class GenericBuilder<T> {private final Supplier<T> supplier;private GenericBuilder(Supplier<T> supplier) {this.supplier = supplier;}public static <T> GenericBuilder<T> of(Supplier<T> supplier) {return new GenericBuilder<>(supplier);}public <P> GenericBuilder<T> with(BiConsumer<T, P> consumer, P value) {return new GenericBuilder<>(() -> {T object = supplier.get();consumer.accept(object, value);return object;});}public T build() {return supplier.get();}
} Post post = GenericBuilder.of(Post::new).with(Post::setTitle, "builder pattern").with(Post::setAuthor, "dachuili").with(Post::setLabel, "design patterns").with(Post::setContent, "classic/generic/lombok builder pattern").build();Builder模式与抽象类
以下代码来自 Joshua Bloch 的《Effective Java》一书，抽象类有自己的Builder方法， 实现类有自己的 Builder方法。
首先是我们的抽象类 Pizza，定义了一些 topping，也就是Pizza上铺的食材（火腿、蘑菇、洋葱、辣椒、香肠之类的）。
public abstract class Pizza {public enum Topping { HAM, MUSHROOM, ONION, PEPPER, SAUSAGE }final Set<Topping> toppings;Pizza(Builder<?> builder) {toppings = builder.toppings.clone();}abstract static class Builder<T extends Builder<T>> {EnumSet<Topping> toppings = EnumSet.noneOf(Topping.class);public T addTopping(Topping topping) {toppings.add(Objects.requireNonNull(topping));return self();}// Subclasses must override this method to return "this"protected abstract T self();abstract Pizza build();}
}其中一个实现类是 NyPizza 和 自己的 Builder 实现类，New York Pizza 有不同的size。
public class NyPizza extends Pizza{public enum Size { SMALL, MEDIUM, LARGE }private final Size size;public static class Builder extends Pizza.Builder<Builder>{private final Size size;public Builder(Size size) {this.size = Objects.requireNonNull(size);}@Override public NyPizza build() {return new NyPizza(this);}@Override protected Builder self() { return this; }}private NyPizza(Builder builder) {super(builder);size = builder.size;}
}另一个实现类是 Calzone，搜了下就是这个菜盒子。🤣
不区分大小，但有个属性 sauceInside 代表“是否内部有酱汁”。
public class Calzone extends Pizza{private final boolean sauceInside;public static class Builder extends Pizza.Builder<Builder>{private boolean sauceInside = false; // Defaultpublic Builder sauceInside() {sauceInside = true;return this;}@Override public Calzone build() {return new Calzone(this);}@Override protected Builder self() { return this; }}private Calzone(Builder builder) {super(builder);sauceInside = builder.sauceInside;}
}创建过程如下：
NyPizza pizza = new NyPizza.Builder(SMALL).addTopping(SAUSAGE).addTopping(ONION).build();Calzone calzone = new Calzone.Builder().addTopping(HAM).sauceInside().build();
