1.定义以及UML建模图:
将一个复杂的对象的构建与它的表示分离，是的同样的构建过程可以创建不同的表示.
Builder设计模式.png
2.使用场景：
多个部件或者零件，都可以装配到一个对象中，但是产生的运行结果又相同
产品类非常复杂，或者产品类中调用顺序不同产生了不同的作用
初始化一个对象特别复杂，如使用多个构造方法，或者说有很多参数，并且都有默认值时。
3.核心类
（1） 抽象产品类 computer
（2） Builder 抽象Builder，规范产品的组建，一般是由子类实现具体的组建过程。
（3）MacbookBuilder 具体的Builder类，具体的创建对象的类。
（4） Directror 统一组建过程
以组装一台计算机为主，主要进行组装主机，设置操作系统，设置显示器。
4.具体使用：
4.1 计算机抽象类，Computer
public abstract class Computer {
protected String mBoard;
protected String mDisplay;
protected String mOs;
protected Computer(){
}
public void setBoard(String board){
mBoard=board;
}
public void setDisplay(String display) {
this.mDisplay = display;
}
public abstract void setOs() ;
@Override
public String toString() {
return "Computer{" +
"mBoard='" + mBoard + '\'' +
", mDisplay='" + mDisplay + '\'' +
", mOs='" + mOs + '\'' +
'}';
}
}
4.2 具体的MacBook ,具体产品类
public class MacBook extends Computer{
protected MacBook() {
}
@Override
public void setOs() {
mOs="Mac OS X 12";
}
}
4.3 抽象Builder
public abstract class Builder {
abstract void buildBoard(String board);
abstract void buildDisplay(String display);
abstract void buildOs();
abstract Computer build();
}
4.4 具体的Builder
public class MacBookBuilder extends Builder {
private Computer mComputer=new MacBook();
@Override
void buildBoard(String board) {
mComputer.setBoard(board);
}
@Override
void buildDisplay(String display) {
mComputer.setDisplay(display);
}
@Override
void buildOs() {
mComputer.setOs();
}
@Override
Computer build() {
return mComputer;
}
}
4.5 Director类，负责具体的构造Computer
public class Director {
Builder mBuilser=null;
public Director(Builder builer) {
this.mBuilser = builer;
}
public void construct(String board,String display){
mBuilser.buildDisplay(display);
mBuilser.buildBoard(board);
mBuilser.buildOs();
}
}
4.6 具体使用
public class Test {
public static void main(String[] args){
Builder builder=new MacBookBuilder();
Director pcDirector=new Director(builder);
pcDirector.construct("英特尔主板","Retina显示器");
Computer computer = builder.build();
System.out.println(computer.toString());
}
}
5.总结
为了灵活构造复杂对象，该对象会有多个成员变量，在外部调用的时候，不需要或者不方便一次性创建出所有的成员变量，在这种情况下，使用多个构造方法去构建对象，很难维护，这时候Builder设计模式解决这个问题，进行buid()方法中创建对象，并且将builder传入，该builder中，维护了传入对象的成员变量。
6. Okhttp中的Request对象使用的Builder设计模式分析
如果你研究过Okhttp中的源码，对Request对象应该很熟悉，接下来看看Request是如何实现链式一句话创建的。
以下是okhttp中的request对象的源码，后面进行简单的分析其设计模式。
/*
* Copyright (C) 2013 Square, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package okhttp3;
import java.net.URL;
import java.util.List;
import javax.annotation.Nullable;
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 @Nullable 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 @Nullable String header(String name) {
return headers.get(name);
}
public List<String> headers(String name) {
return headers.values(name);
}
public @Nullable RequestBody body() {
return body;
}
public Object tag() {
return tag;
}
public Builder newBuilder() {
return new Builder(this);
}
/**
* Returns the cache control directives for this response. This is never null, even if this
* response contains no {@code Cache-Control} header.
*/
public CacheControl cacheControl() {
CacheControl result = cacheControl;
return result != null ? result : (cacheControl = CacheControl.parse(headers));
}
public boolean isHttps() {
return url.isHttps();
}
@Override public String toString() {
return "Request{method="
+ method
+ ", url="
+ url
+ ", tag="
+ (tag != this ? tag : null)
+ '}';
}
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;
}
/**
* Sets the URL target of this request.
*
* @throws IllegalArgumentException if {@code url} is not a valid HTTP or HTTPS URL. Avoid this
* exception by calling {@link HttpUrl#parse}; it returns null for invalid URLs.
*/
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);
}
/**
* Sets the URL target of this request.
*
* @throws IllegalArgumentException if the scheme of {@code url} is not {@code http} or {@code
* https}.
*/
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);
}
/**
* Sets the header named {@code name} to {@code value}. If this request already has any headers
* with that name, they are all replaced.
*/
public Builder header(String name, String value) {
headers.set(name, value);
return this;
}
/**
* Adds a header with {@code name} and {@code value}. Prefer this method for multiply-valued
* headers like "Cookie".
*
* <p>Note that for some headers including {@code Content-Length} and {@code Content-Encoding},
* OkHttp may replace {@code value} with a header derived from the request body.
*/
public Builder addHeader(String name, String value) {
headers.add(name, value);
return this;
}
/** Removes all headers named {@code name} on this builder. */
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;
}
/**
* Sets this request's {@code Cache-Control} header, replacing any cache control headers already
* present. If {@code cacheControl} doesn't define any directives, this clears this request's
* cache-control headers.
*/
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(@Nullable 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, @Nullable 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;
}
/**
* Attaches {@code tag} to the request. It can be used later to cancel the request. If the tag
* is unspecified or null, the request is canceled by using the request itself as the tag.
*/
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);
}
}
}
在项目中我们构建Request对象如下:
Request request=new Request.Builder()
.url("http://www.baidu.com")
.get()
.addHeader("Cache-Control", "public, max-age=60")
.build();
通过一句话，将请求方法，请求地址，设置请求头信息，都已经做好了。
- 简单分析一下其原理
- 具体的产品类，即需要构造的类 Request
public final class Request {
final HttpUrl url;
final String method;
final Headers headers;
final @Nullable 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 @Nullable String header(String name) {
return headers.get(name);
}
public List<String> headers(String name) {
return headers.values(name);
}
public @Nullable RequestBody body() {
return body;
}
public Object tag() {
return tag;
}
public Builder newBuilder() {
return new Builder(this);
}
/**
* Returns the cache control directives for this response. This is never null, even if this
* response contains no {@code Cache-Control} header.
*/
public CacheControl cacheControl() {
CacheControl result = cacheControl;
return result != null ? result : (cacheControl = CacheControl.parse(headers));
}
public boolean isHttps() {
return url.isHttps();
}
@Override public String toString() {
return "Request{method="
+ method
+ ", url="
+ url
+ ", tag="
+ (tag != this ? tag : null)
+ '}';
}
- 具体Builder类：
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;
}
/**
* Sets the URL target of this request.
*
* @throws IllegalArgumentException if {@code url} is not a valid HTTP or HTTPS URL. Avoid this
* exception by calling {@link HttpUrl#parse}; it returns null for invalid URLs.
*/
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);
}
/**
* Sets the URL target of this request.
*
* @throws IllegalArgumentException if the scheme of {@code url} is not {@code http} or {@code
* https}.
*/
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);
}
/**
* Sets the header named {@code name} to {@code value}. If this request already has any headers
* with that name, they are all replaced.
*/
public Builder header(String name, String value) {
headers.set(name, value);
return this;
}
/**
* Adds a header with {@code name} and {@code value}. Prefer this method for multiply-valued
* headers like "Cookie".
*
* <p>Note that for some headers including {@code Content-Length} and {@code Content-Encoding},
* OkHttp may replace {@code value} with a header derived from the request body.
*/
public Builder addHeader(String name, String value) {
headers.add(name, value);
return this;
}
/** Removes all headers on this builder and adds {@code headers}. */
public Builder headers(Headers headers) {
this.headers = headers.newBuilder();
return this;
}
/**
* Sets this request's {@code Cache-Control} header, replacing any cache control headers already
* present. If {@code cacheControl} doesn't define any directives, this clears this request's
* cache-control headers.
*/
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);
}
/**
* Attaches {@code tag} to the request. It can be used later to cancel the request. If the tag
* is unspecified or null, the request is canceled by using the request itself as the tag.
*/
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);
}
}
- 可以看到Okhttp中Request的使用到的Builder设计模式更灵活，没有进行抽象类约束，因为在Okhttp中，只生成request这一个对象，所以直接使用具体Builder和具体的产品直接进行构建组建自己。而且在构建完部件以后，直接返回该Builder对象，方便链式调用，也达到了一句话创建复杂request的对象的过程。
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