漫画：什么是 “建造者模式” ？

拼搏现实的明天。 2022-09-04 01:47 100阅读 0赞

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**—————  第二天  —————**

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首先，我们来定义一个Product类：

public class Product {
        ArrayList<String> parts = new ArrayList<String>();
    
        public void add(String part) {
            parts.add(part);
        }
    
        public void show() {
            System.out.println(parts);
        }
    }

接下来，我们定义抽象的Builder类：

public abstract class Builder {
        public abstract void buildPartA();
        public abstract void buildPartB();
        public abstract Product getResult() ;
    }

然后，是具体的Builder实现类：

public class ConcreteBuilder extends Builder {
        private Product product = new Product();
        public Product getResult() {
            return product;
        }
        @Override
        public void buildPartA() {
            product.add("构建产品的上半部分");
        }
    
        @Override
        public void buildPartB() {
            product.add("构建产品的下半部分");
        }
    }

在Builder类之外，则是Director类来控制Builder的生产过程：

public class Director {
        private Builder builder;
    
        public Director(Builder builder) {
            this.builder = builder;
        }
    
        public void construct() {
            builder.buildPartA();
            builder.buildPartB();
        }
    }

最后，是客户端的测试代码：

Builder builder = new ConcreteBuilder();
    Director director = new Director(builder);
    director.construct();
    Product product = builder.getResult();
    product.show();

我们来看一下运行的结果：

[构建产品的上半部分, 构建产品的下半部分]

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// 默认采用Builder进行建造
    public OkHttpClient() {
      this(new Builder());
    }
    
    // 由builder配置分发器、代理、协议以及自定义拦截器等
    OkHttpClient(Builder builder) {
      this.dispatcher = builder.dispatcher;
      this.proxy = builder.proxy;
      this.protocols = builder.protocols;
      /** 省略大段代码 */
      boolean isTLS = false;
      for (ConnectionSpec spec : connectionSpecs) {
        isTLS = isTLS || spec.isTls();
      }
      /** 省略大段代码. */
      if (interceptors.contains(null)) {
        throw new IllegalStateException("Null interceptor: " + interceptors);
      }
      if (networkInterceptors.contains(null)) {
        throw new IllegalStateException("Null network interceptor: " + networkInterceptors);
      }
    }
    public static final class Builder {
    
      public Builder() {
    // 分发器、协议、代理的默认参数
        dispatcher = new Dispatcher();
        protocols = DEFAULT_PROTOCOLS;
        proxySelector = ProxySelector.getDefault();
        if (proxySelector == null) {
          proxySelector = new NullProxySelector();
        }
      }
    
      Builder(OkHttpClient okHttpClient) {
        // 反向配置分发器、代理、协议
        this.dispatcher = okHttpClient.dispatcher; 
        this.proxy = okHttpClient.proxy; 
        this.protocols = okHttpClient.protocols;
    // 新增所有自定义拦截器和自定义网络拦截器
        this.interceptors.addAll(okHttpClient.interceptors);
        this.networkInterceptors.addAll(okHttpClient.networkInterceptors);
      }
    
      // 配置代理
      public Builder proxy(@Nullable Proxy proxy) {
        this.proxy = proxy;
        return this;
      }
      // 向拦截器链中增加自定义拦截器
      public Builder addInterceptor(Interceptor interceptor) {
        if (interceptor == null) throw new IllegalArgumentException("interceptor == null");
        interceptors.add(interceptor);
        return this;
      }
      // 最后是build()方法，生成OkHttpClient对象
      public OkHttpClient build() {
        return new OkHttpClient(this);
      }
    }

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/**将指定的字符串追加到此字符序列*/
    @Override
    public StringBuilder append(CharSequence s) {
        super.append(s);// 实现过程略
        return this;
    }
    /**将此字符序列用其反转形式取代*/
    @Override
    public StringBuilder reverse() {
        super.reverse();// 实现过程略
        return this;
    }

下面让我们来编写一下测试代码：

StringBuilder sb = new StringBuilder("若为自由故");
    sb.append("只要主义真");
    sb.reverse();
    System.out.println(sb);
    StringBuilder sb1 = new StringBuilder("若为自由故");
    sb1.reverse();
    sb1.append("只要主义真");
    System.out.println(sb1);

测试结果如下：

System.out: 真义主要只故由自为若
    System.out: 故由自为若只要主义真

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建造者模式与简单工程模式的区别，在于建造者模式多出一个Builder类，使得创建对象的灵活性大大增加，适用于如下场景：

（1）创建一个对象，多个同样的方法的调用顺序不同，产生的结果不同

（2）创建一个对象，特别复杂，参数多，而且很多参数都有默认值

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System.out.println("Hello World");

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短短一行代码，背后有什么样的机制呢？

Java的编译原理，是将Hello.java编译成能被VM理解的Hello.class，然后再转化为能被不同硬件设备理解的bytecode进而执行的。

著名的字节码增强框架ASM，就是在Hello.java编译成Hello.class时可以读取并分析类信息、改变类行为、增强类功能甚至生成新的类的bytecode分析和操作框架。

我们来看一下相关的代码，代码当中的mv，来自ASM框架的MethodVisitor接口。

// 访问System类的类型为PrintSystem类型的静态变量out
    mv.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
    // 访问常量池中的数据"Hello World"
    mv.visitLdcInsn("Hello World");
    // 调用PrintStream类的println()方法并把刚才获取到的对象当做String类型的参数传入
    mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);

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