模式定义

将一个复杂对象的构建与它的表示分离，使得同样的构建过程可以创建不同的表示。

建造者模式是一步一步创建一个复杂的对象，它允许用户只通过指定复杂对象的类型和内容就可以构建它们，用户不需要知道内部的具体构建细节。建造者模式属于对象创建型模式。建造者模式又称为生成器模式。

模式结构

代码示例

OkHttp 源码中的Builder模式

** 本篇中 OkHttp 版本为 3.6.0 **

Builder模式在 OkHttp中得到了大量应用，例如OkHttpClient、Request、FormBody、MultipartBody等对象构造。

使用OkHttp 实现表单提交功能如下：

  private final OkHttpClient client = new OkHttpClient();

  public void run() throws Exception {
    RequestBody formBody = new FormBody.Builder()
        .add("search", "Jurassic Park")
        .build();
    Request request = new Request.Builder()
        .url("https://en.wikipedia.org/w/index.php")
        .post(formBody)
        .build();

    Response response = client.newCall(request).execute();
    if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);

    System.out.println(response.body().string());
  }

接下来我们看一下 okhttp3.Request 的源码：

package okhttp3;

import java.net.URL;
import java.util.List;
import okhttp3.internal.Util;
import okhttp3.internal.http.HttpMethod;

/**
 * An HTTP request. Instances of this class are immutable if their {@link #body} is null or itself
 * immutable.
 */
public final class Request {
  final HttpUrl url;
  final String method;
  final Headers headers;
  final RequestBody body;
  final Object tag;

  private volatile CacheControl cacheControl; // Lazily initialized.

  Request(Builder builder) {
    this.url = builder.url;
    this.method = builder.method;
    this.headers = builder.headers.build();
    this.body = builder.body;
    this.tag = builder.tag != null ? builder.tag : this;
  }

  public HttpUrl url() {
    return url;
  }

  public String method() {
    return method;
  }

  public Headers headers() {
    return headers;
  }

  public String header(String name) {
    return headers.get(name);
  }

  public List<String> headers(String name) {
    return headers.values(name);
  }

  public RequestBody body() {
    return body;
  }

  public Object tag() {
    return tag;
  }

  public Builder newBuilder() {
    return new Builder(this);
  }

  public CacheControl cacheControl() {
    CacheControl result = cacheControl;
    return result != null ? result : (cacheControl = CacheControl.parse(headers));
  }

  public boolean isHttps() {
    return url.isHttps();
  }

  /** Builder 模式体现 **/
  public static class Builder {
    HttpUrl url;
    String method;
    Headers.Builder headers;
    RequestBody body;
    Object tag;

    public Builder() {
      this.method = "GET";
      this.headers = new Headers.Builder();
    }

    Builder(Request request) {
      this.url = request.url;
      this.method = request.method;
      this.body = request.body;
      this.tag = request.tag;
      this.headers = request.headers.newBuilder();
    }

    public Builder url(HttpUrl url) {
      if (url == null) throw new NullPointerException("url == null");
      this.url = url;
      return this;
    }

    public Builder url(String url) {
      if (url == null) throw new NullPointerException("url == null");

      // Silently replace web socket URLs with HTTP URLs.
      if (url.regionMatches(true, 0, "ws:", 0, 3)) {
        url = "http:" + url.substring(3);
      } else if (url.regionMatches(true, 0, "wss:", 0, 4)) {
        url = "https:" + url.substring(4);
      }

      HttpUrl parsed = HttpUrl.parse(url);
      if (parsed == null) throw new IllegalArgumentException("unexpected url: " + url);
      return url(parsed);
    }

    public Builder url(URL url) {
      if (url == null) throw new NullPointerException("url == null");
      HttpUrl parsed = HttpUrl.get(url);
      if (parsed == null) throw new IllegalArgumentException("unexpected url: " + url);
      return url(parsed);
    }

    public Builder header(String name, String value) {
      headers.set(name, value);
      return this;
    }

    public Builder addHeader(String name, String value) {
      headers.add(name, value);
      return this;
    }

    public Builder removeHeader(String name) {
      headers.removeAll(name);
      return this;
    }

    /** Removes all headers on this builder and adds {@code headers}. */
    public Builder headers(Headers headers) {
      this.headers = headers.newBuilder();
      return this;
    }

    public Builder cacheControl(CacheControl cacheControl) {
      String value = cacheControl.toString();
      if (value.isEmpty()) return removeHeader("Cache-Control");
      return header("Cache-Control", value);
    }

    public Builder get() {
      return method("GET", null);
    }

    public Builder head() {
      return method("HEAD", null);
    }

    public Builder post(RequestBody body) {
      return method("POST", body);
    }

    public Builder delete(RequestBody body) {
      return method("DELETE", body);
    }

    public Builder delete() {
      return delete(Util.EMPTY_REQUEST);
    }

    public Builder put(RequestBody body) {
      return method("PUT", body);
    }

    public Builder patch(RequestBody body) {
      return method("PATCH", body);
    }

    public Builder method(String method, RequestBody body) {
      if (method == null) throw new NullPointerException("method == null");
      if (method.length() == 0) throw new IllegalArgumentException("method.length() == 0");
      if (body != null && !HttpMethod.permitsRequestBody(method)) {
        throw new IllegalArgumentException("method " + method + " must not have a request body.");
      }
      if (body == null && HttpMethod.requiresRequestBody(method)) {
        throw new IllegalArgumentException("method " + method + " must have a request body.");
      }
      this.method = method;
      this.body = body;
      return this;
    }

    public Builder tag(Object tag) {
      this.tag = tag;
      return this;
    }

    public Request build() {
      if (url == null) throw new IllegalStateException("url == null");
      return new Request(this);
    }
  }
}