java - 继承 Java Builder 类

Question

给出Dobbs 博士的这篇文章，特别是 Builder 模式，我们如何处理子类化 Builder 的情况？以我们想要子类化以添加 GMO 标签的示例的缩减版本为例，一个简单的实现将是：

public class NutritionFacts {                                                                                                    

    private final int calories;                                                                                                  

    public static class Builder {                                                                                                
        private int calories = 0;                                                                                                

        public Builder() {}                                                                                                      

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

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

    protected NutritionFacts(Builder builder) {                                                                                  
        calories = builder.calories;                                                                                             
    }                                                                                                                            
}

子类：

public class GMOFacts extends NutritionFacts {                                                                                   

    private final boolean hasGMO;                                                                                                

    public static class Builder extends NutritionFacts.Builder {                                                                 

        private boolean hasGMO = false;                                                                                          

        public Builder() {}                                                                                                      

        public Builder GMO(boolean val) { hasGMO = val; return this; }                                                           

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

    protected GMOFacts(Builder builder) {                                                                                        
        super(builder);                                                                                                          
        hasGMO = builder.hasGMO;                                                                                                 
    }                                                                                                                            
}

现在，我们可以编写如下代码：

GMOFacts.Builder b = new GMOFacts.Builder();
b.GMO(true).calories(100);

但是，如果我们弄错了顺序，一切都会失败：

GMOFacts.Builder b = new GMOFacts.Builder();
b.calories(100).GMO(true);

问题当然是NutritionFacts.Builder返回 a NutritionFacts.Builder，而不是 a GMOFacts.Builder，那么我们如何解决这个问题，或者是否有更好的 Pattern 可以使用？

注意：this answer to a similar question提供了我上面的课程；我的问题是关于确保构建器调用顺序正确的问题。

score 190 · Accepted Answer

您可以使用泛型解决它。我认为这被称为“奇怪地重复出现的通用模式”

使基类构建器方法的返回类型成为通用参数。

public class NutritionFacts {

    private final int calories;

    public static class Builder<T extends Builder<T>> {

        private int calories = 0;

        public Builder() {}

        public T calories(int val) {
            calories = val;
            return (T) this;
        }

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

    protected NutritionFacts(Builder<?> builder) {
        calories = builder.calories;
    }
}

现在使用派生类构建器作为通用参数实例化基础构建器。

public class GMOFacts extends NutritionFacts {

    private final boolean hasGMO;

    public static class Builder extends NutritionFacts.Builder<Builder> {

        private boolean hasGMO = false;

        public Builder() {}

        public Builder GMO(boolean val) {
            hasGMO = val;
            return this;
        }

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

    protected GMOFacts(Builder builder) {
        super(builder);
        hasGMO = builder.hasGMO;
    }
}

score 48 · Accepted Answer

只是为了记录，摆脱

unchecked or unsafe operations警告

对于return (T) this;@dimadima 和@Thomas N. 谈论的声明，以下解决方案适用于某些情况。

abstract创建声明泛型类型的构建器（在T extends Builder本例中）并声明protected abstract T getThis()抽象方法，如下所示：

public abstract static class Builder<T extends Builder<T>> {

    private int calories = 0;

    public Builder() {}

    /** The solution for the unchecked cast warning. */
    public abstract T getThis();

    public T calories(int val) {
        calories = val;

        // no cast needed
        return getThis();
    }

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

有关详细信息，请参阅http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#FAQ205。

score 22 · Accepted Answer

根据博客文章，这种方法要求所有非叶类都是抽象的，并且所有叶类都必须是最终的。

public abstract class TopLevel {
    protected int foo;
    protected TopLevel() {
    }
    protected static abstract class Builder
        <T extends TopLevel, B extends Builder<T, B>> {
        protected T object;
        protected B thisObject;
        protected abstract T createObject();
        protected abstract B thisObject();
        public Builder() {
            object = createObject();
            thisObject = thisObject();
        }
        public B foo(int foo) {
            object.foo = foo;
            return thisObject;
        }
        public T build() {
            return object;
        }
    }
}

然后，您有一些中间类来扩展此类及其构建器，以及您需要的更多：

public abstract class SecondLevel extends TopLevel {
    protected int bar;
    protected static abstract class Builder
        <T extends SecondLevel, B extends Builder<T, B>> extends TopLevel.Builder<T, B> {
        public B bar(int bar) {
            object.bar = bar;
            return thisObject;
        }
    }
}

最后是一个具体的叶类，它可以按任何顺序调用其任何父级的所有构建器方法：

public final class LeafClass extends SecondLevel {
    private int baz;
    public static final class Builder extends SecondLevel.Builder<LeafClass,Builder> {
        protected LeafClass createObject() {
            return new LeafClass();
        }
        protected Builder thisObject() {
            return this;
        }
        public Builder baz(int baz) {
            object.baz = baz;
            return thisObject;
        }
    }
}

然后，您可以从层次结构中的任何类以任何顺序调用方法：

public class Demo {
    LeafClass leaf = new LeafClass.Builder().baz(2).foo(1).bar(3).build();
}

score 7 · Accepted Answer

您也可以覆盖该calories()方法，并让它返回扩展构建器。这是因为 Java 支持协变返回类型而编译的。

public class GMOFacts extends NutritionFacts {
    private final boolean hasGMO;
    public static class Builder extends NutritionFacts.Builder {
        private boolean hasGMO = false;
        public Builder() {
        }
        public Builder GMO(boolean val)
        { hasGMO = val; return this; }
        public Builder calories(int val)
        { super.calories(val); return this; }
        public GMOFacts build() {
            return new GMOFacts(this);
        }
    }
    [...]
}

score 3 · Accepted Answer

还有另一种根据Builder模式创建类的方法，它符合“优先组合优于继承”。

定义一个接口，该父类Builder将继承：

public interface FactsBuilder<T> {

    public T calories(int val);
}

的实现NutritionFacts几乎相同（除了Builder实现'FactsBuilder'接口）：

public class NutritionFacts {

    private final int calories;

    public static class Builder implements FactsBuilder<Builder> {
        private int calories = 0;

        public Builder() {
        }

        @Override
        public Builder calories(int val) {
            return this;
        }

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

    protected NutritionFacts(Builder builder) {
        calories = builder.calories;
    }
}

子类的Builder应该扩展相同的接口（不同的通用实现除外）：

public static class Builder implements FactsBuilder<Builder> {
    NutritionFacts.Builder baseBuilder;

    private boolean hasGMO = false;

    public Builder() {
        baseBuilder = new NutritionFacts.Builder();
    }

    public Builder GMO(boolean val) {
        hasGMO = val;
        return this;
    }

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

    @Override
    public Builder calories(int val) {
        baseBuilder.calories(val);
        return this;
    }
}

请注意，这NutritionFacts.Builder是内部的一个字段GMOFacts.Builder（称为baseBuilder）。从FactsBuilder接口调用baseBuilder的同名方法实现的方法：

@Override
public Builder calories(int val) {
    baseBuilder.calories(val);
    return this;
}

的构造函数也有很大的变化GMOFacts(Builder builder)。构造函数中对父类构造函数的第一次调用应该传递适当的NutritionFacts.Builder：

protected GMOFacts(Builder builder) {
    super(builder.baseBuilder);
    hasGMO = builder.hasGMO;
}

类的完整实现GMOFacts：

public class GMOFacts extends NutritionFacts {

    private final boolean hasGMO;

    public static class Builder implements FactsBuilder<Builder> {
        NutritionFacts.Builder baseBuilder;

        private boolean hasGMO = false;

        public Builder() {
        }

        public Builder GMO(boolean val) {
            hasGMO = val;
            return this;
        }

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

        @Override
        public Builder calories(int val) {
            baseBuilder.calories(val);
            return this;
        }
    }

    protected GMOFacts(Builder builder) {
        super(builder.baseBuilder);
        hasGMO = builder.hasGMO;
    }
}

score 3 · Accepted Answer

多重构建器继承的完整 3 级示例如下所示：

（对于具有构建器的复制构造函数的版本，请参见下面的第二个示例）

第一级 - 父级（可能是抽象的）

import lombok.ToString;

@ToString
@SuppressWarnings("unchecked")
public abstract class Class1 {
    protected int f1;

    public static class Builder<C extends Class1, B extends Builder<C, B>> {
        C obj;

        protected Builder(C constructedObj) {
            this.obj = constructedObj;
        }

        B f1(int f1) {
            obj.f1 = f1;
            return (B)this;
        }

        C build() {
            return obj;
        }
    }
}

第二级

import lombok.ToString;

@ToString(callSuper=true)
@SuppressWarnings("unchecked")
public class Class2 extends Class1 {
    protected int f2;

    public static class Builder<C extends Class2, B extends Builder<C, B>> extends Class1.Builder<C, B> {
        public Builder() {
            this((C) new Class2());
        }

        protected Builder(C obj) {
            super(obj);
        }

        B f2(int f2) {
            obj.f2 = f2;
            return (B)this;
        }
    }
}

三级

import lombok.ToString;

@ToString(callSuper=true)
@SuppressWarnings("unchecked")
public class Class3 extends Class2 {
    protected int f3;

    public static class Builder<C extends Class3, B extends Builder<C, B>> extends Class2.Builder<C, B> {
        public Builder() {
            this((C) new Class3());
        }

        protected Builder(C obj) {
            super(obj);
        }

        B f3(int f3) {
            obj.f3 = f3;
            return (B)this;
        }
    }
}

以及使用示例

public class Test {
    public static void main(String[] args) {
        Class2 b1 = new Class2.Builder<>().f1(1).f2(2).build();
        System.out.println(b1);
        Class2 b2 = new Class2.Builder<>().f2(2).f1(1).build();
        System.out.println(b2);

        Class3 c1 = new Class3.Builder<>().f1(1).f2(2).f3(3).build();
        System.out.println(c1);
        Class3 c2 = new Class3.Builder<>().f3(3).f1(1).f2(2).build();
        System.out.println(c2);
        Class3 c3 = new Class3.Builder<>().f3(3).f2(2).f1(1).build();
        System.out.println(c3);
        Class3 c4 = new Class3.Builder<>().f2(2).f3(3).f1(1).build();
        System.out.println(c4);
    }
}

更长的版本，具有构建器的复制构造函数：

第一级 - 父级（可能是抽象的）

import lombok.ToString;

@ToString
@SuppressWarnings("unchecked")
public abstract class Class1 {
    protected int f1;

    public static class Builder<C extends Class1, B extends Builder<C, B>> {
        C obj;

        protected void setObj(C obj) {
            this.obj = obj;
        }

        protected void copy(C obj) {
            this.f1(obj.f1);
        }

        B f1(int f1) {
            obj.f1 = f1;
            return (B)this;
        }

        C build() {
            return obj;
        }
    }
}

第二级

import lombok.ToString;

@ToString(callSuper=true)
@SuppressWarnings("unchecked")
public class Class2 extends Class1 {
    protected int f2;

    public static class Builder<C extends Class2, B extends Builder<C, B>> extends Class1.Builder<C, B> {
        public Builder() {
            setObj((C) new Class2());
        }

        public Builder(C obj) {
            this();
            copy(obj);
        }

        @Override
        protected void copy(C obj) {
            super.copy(obj);
            this.f2(obj.f2);
        }

        B f2(int f2) {
            obj.f2 = f2;
            return (B)this;
        }
    }
}

三级

import lombok.ToString;

@ToString(callSuper=true)
@SuppressWarnings("unchecked")
public class Class3 extends Class2 {
    protected int f3;

    public static class Builder<C extends Class3, B extends Builder<C, B>> extends Class2.Builder<C, B> {
        public Builder() {
            setObj((C) new Class3());
        }

        public Builder(C obj) {
            this();
            copy(obj);
        }

        @Override
        protected void copy(C obj) {
            super.copy(obj);
            this.f3(obj.f3);
        }

        B f3(int f3) {
            obj.f3 = f3;
            return (B)this;
        }
    }
}

以及使用示例

public class Test {
    public static void main(String[] args) {
        Class3 c4 = new Class3.Builder<>().f2(2).f3(3).f1(1).build();
        System.out.println(c4);

        // Class3 builder copy
        Class3 c42 = new Class3.Builder<>(c4).f2(12).build();
        System.out.println(c42);
        Class3 c43 = new Class3.Builder<>(c42).f2(22).f1(11).build();
        System.out.println(c43);
        Class3 c44 = new Class3.Builder<>(c43).f3(13).f1(21).build();
        System.out.println(c44);
    }
}

score 2 · Accepted Answer

如果你不想把眼睛放在一个或三个尖括号上，或者可能感觉不到你……嗯……我的意思是……咳……你团队的其他人很快就会好奇地理解重复泛型模式，您可以这样做：

public class TestInheritanceBuilder {
  public static void main(String[] args) {
    SubType.Builder builder = new SubType.Builder();
    builder.withFoo("FOO").withBar("BAR").withBaz("BAZ");
    SubType st = builder.build();
    System.out.println(st.toString());
    builder.withFoo("BOOM!").withBar("not getting here").withBaz("or here");
  }
}

支持

public class SubType extends ParentType {
  String baz;
  protected SubType() {}

  public static class Builder extends ParentType.Builder {
    private SubType object = new SubType();

    public Builder withBaz(String baz) {
      getObject().baz = baz;
      return this;
    }

    public Builder withBar(String bar) {
      super.withBar(bar);
      return this;
    }

    public Builder withFoo(String foo) {
      super.withFoo(foo);
      return this;
    }

    public SubType build() {
      // or clone or copy constructor if you want to stamp out multiple instances...
      SubType tmp = getObject();
      setObject(new SubType());
      return tmp;
    }

    protected SubType getObject() {
      return object;
    }

    private void setObject(SubType object) {
      this.object = object;
    }
  }

  public String toString() {
    return "SubType2{" +
        "baz='" + baz + '\'' +
        "} " + super.toString();
  }
}

和父类型：

public class ParentType {
  String foo;
  String bar;

  protected ParentType() {}

  public static class Builder {
    private ParentType object = new ParentType();

    public ParentType object() {
      return getObject();
    }

    public Builder withFoo(String foo) {
      if (!"foo".equalsIgnoreCase(foo)) throw new IllegalArgumentException();
      getObject().foo = foo;
      return this;
    }

    public Builder withBar(String bar) {
      getObject().bar = bar;
      return this;
    }

    protected ParentType getObject() {
      return object;
    }

    private void setObject(ParentType object) {
      this.object = object;
    }

    public ParentType build() {
      // or clone or copy constructor if you want to stamp out multiple instances...
      ParentType tmp = getObject();
      setObject(new ParentType());
      return tmp;
    }
  }

  public String toString() {
    return "ParentType2{" +
        "foo='" + foo + '\'' +
        ", bar='" + bar + '\'' +
        '}';
  }
}

关键点：

在构建器中封装对象，以便继承防止您在父类型中保存的对象上设置字段
对 super 的调用确保添加到超类型构建器方法的逻辑（如果有）保留在子类型中。
不利的一面是在父类中创建虚假对象......但请参阅下面的清理方法
向上一目了然更容易理解，并且没有冗长的构造函数传输属性。
如果您有多个线程访问您的构建器对象......我想我很高兴我不是你:)。

编辑：

我找到了一种绕过虚假对象创建的方法。首先将其添加到每个构建器：

private Class whoAmI() {
  return new Object(){}.getClass().getEnclosingMethod().getDeclaringClass();
}

然后在每个构建器的构造函数中：

  if (whoAmI() == this.getClass()) {
    this.obj = new ObjectToBuild();
  }

成本是new Object(){}匿名内部类的额外类文件

score 1 · Accepted Answer

您可以做的一件事是在每个类中创建一个静态工厂方法：

NutritionFacts.newBuilder()
GMOFacts.newBuilder()

然后，此静态工厂方法将返回适当的构建器。你可以有一个GMOFacts.Builder扩展 a NutritionFacts.Builder，这不是问题。这里的问题将是处理可见性......

score 0 · Accepted Answer

我创建了一个父抽象通用构建器类，它接受两个形式类型参数。第一个是 build() 返回的对象类型，第二个是每个可选参数设置器返回的类型。以下是用于说明目的的父类和子类：

// **Parent**
public abstract static class Builder<T, U extends Builder<T, U>> {
    // Required parameters
    private final String name;

    // Optional parameters
    private List<String> outputFields = null;


    public Builder(String pName) {
        name = pName;
    }

    public U outputFields(List<String> pOutFlds) {
        outputFields = new ArrayList<>(pOutFlds);
        return getThis();
    }


    /**
     * This helps avoid "unchecked warning", which would forces to cast to "T" in each of the optional
     * parameter setters..
     * @return
     */
    abstract U getThis();

    public abstract T build();



    /*
     * Getters
     */
    public String getName() {
        return name;
    }
}

 // **Child**
 public static class Builder extends AbstractRule.Builder<ContextAugmentingRule, ContextAugmentingRule.Builder> {
    // Required parameters
    private final Map<String, Object> nameValuePairsToAdd;

    // Optional parameters
    private String fooBar;


    Builder(String pName, Map<String, String> pNameValPairs) {
        super(pName);
        /**
         * Must do this, in case client code (I.e. JavaScript) is re-using
         * the passed in for multiple purposes. Doing {@link Collections#unmodifiableMap(Map)}
         * won't caught it, because the backing Map passed by client prior to wrapping in
         * unmodifiable Map can still be modified.
         */
        nameValuePairsToAdd = new HashMap<>(pNameValPairs);
    }

    public Builder fooBar(String pStr) {
        fooBar = pStr;
        return this;
    }


    @Override
    public ContextAugmentingRule build() {
        try {
            Rule r = new ContextAugmentingRule(this);
            storeInRuleByNameCache(r);
            return (ContextAugmentingRule) r;
        } catch (RuleException e) {
            throw new IllegalArgumentException(e);
        }
    }

    @Override
    Builder getThis() {
        return this;
    }
}

这一款满足了我的需求。

score -1 · Accepted Answer

以下 IEEE 贡献Refined Fluent Builder in Java给出了该问题的全面解决方案。

它将原始问题分解为继承缺陷和准不变性这两个子问题，并展示了如何解决这两个子问题，以支持 Java 中经典构建器模式中代码重用的继承支持。

java - 继承 Java Builder 类

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