一、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。二、角色
1.Product 产品类：建造者最后要构建出来的复杂对象。2.Builder 抽象构建者：为创建一个复杂对象各个组建指定抽象接口，规范产品的组建。
3.ConcreteBuilder 具体建造者：实现抽象类返回的所有方法，写法上一般方法会返回建造者本身，这样调用时候可以一直用“.”号一直调用其它方法了。
4.Director 导演类：执行者，指导构建过程。
三、优点
1.封装性。2.建造者独立，容易扩展，便于控制细节风险。
四、使用场景
1.相同的方法，不同的执行顺序，产生不同的事件结果。2.产品类十分复杂，构造方法或者初始化方法参数过多。
在《Effective Java》书中第二条，就提到“遇到多个构造器参数时要考虑用构建器”，其实这里的构建器就属于建造者模式，只是里面把四个角色都放到具体产品里面了。
五、实例
也许看了以上众多概念后你还是一头雾水，所以我们最好还是来通过一个例子来了解建造者模式的使用与好处。
下面借用 universal-image-loader 里面例子来分析一下：
public final class DisplayImageOptions {
private final int imageResOnLoading;
private final int imageResForEmptyUri;
private final int imageResOnFail;
private final Drawable imageOnLoading;
private final Drawable imageForEmptyUri;
private final Drawable imageOnFail;
private final boolean resetViewBeforeLoading;
private final boolean cacheInMemory;
private final boolean cacheOnDisk;
private final ImageScaleType imageScaleType;
private final Options decodingOptions;
private final int delayBeforeLoading;
private final boolean considerExifParams;
private final Object extraForDownloader;
private final BitmapProcessor preProcessor;
private final BitmapProcessor postProcessor;
private final BitmapDisplayer displayer;
private final Handler handler;
private final boolean isSyncLoading;
private DisplayImageOptions(Builder ：) {
imageResOnLoading = builder.imageResOnLoading;
imageResForEmptyUri = builder.imageResForEmptyUri;
imageResOnFail = builder.imageResOnFail;
imageOnLoading = builder.imageOnLoading;
imageForEmptyUri = builder.imageForEmptyUri;
imageOnFail = builder.imageOnFail;
resetViewBeforeLoading = builder.resetViewBeforeLoading;
cacheInMemory = builder.cacheInMemory;
cacheOnDisk = builder.cacheOnDisk;
imageScaleType = builder.imageScaleType;
decodingOptions = builder.decodingOptions;
delayBeforeLoading = builder.delayBeforeLoading;
considerExifParams = builder.considerExifParams;
extraForDownloader = builder.extraForDownloader;
preProcessor = builder.preProcessor;
postProcessor = builder.postProcessor;
displayer = builder.displayer;
handler = builder.handler;
isSyncLoading = builder.isSyncLoading;
}
public boolean shouldShowImageOnLoading() {
return imageOnLoading != null || imageResOnLoading != 0;
}
public boolean shouldShowImageForEmptyUri() {
return imageForEmptyUri != null || imageResForEmptyUri != 0;
}
public boolean shouldShowImageOnFail() {
return imageOnFail != null || imageResOnFail != 0;
}
public boolean shouldPreProcess() {
return preProcessor != null;
}
public boolean shouldPostProcess() {
return postProcessor != null;
}
public boolean shouldDelayBeforeLoading() {
return delayBeforeLoading > 0;
}
public Drawable getImageOnLoading(Resources res) {
return imageResOnLoading != 0 ? res.getDrawable(imageResOnLoading) : imageOnLoading;
}
public Drawable getImageForEmptyUri(Resources res) {
return imageResForEmptyUri != 0 ? res.getDrawable(imageResForEmptyUri) : imageForEmptyUri;
}
public Drawable getImageOnFail(Resources res) {
return imageResOnFail != 0 ? res.getDrawable(imageResOnFail) : imageOnFail;
}
public boolean isResetViewBeforeLoading() {
return resetViewBeforeLoading;
}
public boolean isCacheInMemory() {
return cacheInMemory;
}
public boolean isCacheOnDisk() {
return cacheOnDisk;
}
public ImageScaleType getImageScaleType() {
return imageScaleType;
}
public Options getDecodingOptions() {
return decodingOptions;
}
public int getDelayBeforeLoading() {
return delayBeforeLoading;
}
public boolean isConsiderExifParams() {
return considerExifParams;
}
public Object getExtraForDownloader() {
return extraForDownloader;
}
public BitmapProcessor getPreProcessor() {
return preProcessor;
}
public BitmapProcessor getPostProcessor() {
return postProcessor;
}
public BitmapDisplayer getDisplayer() {
return displayer;
}
public Handler getHandler() {
return handler;
}
boolean isSyncLoading() {
return isSyncLoading;
}
/**
* Builder for {@link DisplayImageOptions}
*
* @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
*/
public static class Builder {
private int imageResOnLoading = 0;
private int imageResForEmptyUri = 0;
private int imageResOnFail = 0;
private Drawable imageOnLoading = null;
private Drawable imageForEmptyUri = null;
private Drawable imageOnFail = null;
private boolean resetViewBeforeLoading = false;
private boolean cacheInMemory = false;
private boolean cacheOnDisk = false;
private ImageScaleType imageScaleType = ImageScaleType.IN_SAMPLE_POWER_OF_2;
private Options decodingOptions = new Options();
private int delayBeforeLoading = 0;
private boolean considerExifParams = false;
private Object extraForDownloader = null;
private BitmapProcessor preProcessor = null;
private BitmapProcessor postProcessor = null;
private BitmapDisplayer displayer = DefaultConfigurationFactory.createBitmapDisplayer();
private Handler handler = null;
private boolean isSyncLoading = false;
/**
* Stub image will be displayed in {@link com.nostra13.universalimageloader.core.imageaware.ImageAware
* image aware view} during image loading
*
* @param imageRes Stub image resource
* @deprecated Use {@link #showImageOnLoading(int)} instead
*/
@Deprecated
public Builder showStubImage(int imageRes) {
imageResOnLoading = imageRes;
return this;
}
/**
* Incoming image will be displayed in {@link com.nostra13.universalimageloader.core.imageaware.ImageAware
* image aware view} during image loading
*
* @param imageRes Image resource
*/
public Builder showImageOnLoading(int imageRes) {
imageResOnLoading = imageRes;
return this;
}
/**
* Incoming drawable will be displayed in {@link com.nostra13.universalimageloader.core.imageaware.ImageAware
* image aware view} during image loading.
* This option will be ignored if {@link DisplayImageOptions.Builder#showImageOnLoading(int)} is set.
*/
public Builder showImageOnLoading(Drawable drawable) {
imageOnLoading = drawable;
return this;
}
/**
* Incoming image will be displayed in {@link com.nostra13.universalimageloader.core.imageaware.ImageAware
* image aware view} if empty URI (null or empty
* string) will be passed to <b>ImageLoader.displayImage(...)</b> method.
*
* @param imageRes Image resource
*/
public Builder showImageForEmptyUri(int imageRes) {
imageResForEmptyUri = imageRes;
return this;
}
/**
* Incoming drawable will be displayed in {@link com.nostra13.universalimageloader.core.imageaware.ImageAware
* image aware view} if empty URI (null or empty
* string) will be passed to <b>ImageLoader.displayImage(...)</b> method.
* This option will be ignored if {@link DisplayImageOptions.Builder#showImageForEmptyUri(int)} is set.
*/
public Builder showImageForEmptyUri(Drawable drawable) {
imageForEmptyUri = drawable;
return this;
}
/**
* Incoming image will be displayed in {@link com.nostra13.universalimageloader.core.imageaware.ImageAware
* image aware view} if some error occurs during
* requested image loading/decoding.
*
* @param imageRes Image resource
*/
public Builder showImageOnFail(int imageRes) {
imageResOnFail = imageRes;
return this;
}
/**
* Incoming drawable will be displayed in {@link com.nostra13.universalimageloader.core.imageaware.ImageAware
* image aware view} if some error occurs during
* requested image loading/decoding.
* This option will be ignored if {@link DisplayImageOptions.Builder#showImageOnFail(int)} is set.
*/
public Builder showImageOnFail(Drawable drawable) {
imageOnFail = drawable;
return this;
}
/**
* {@link com.nostra13.universalimageloader.core.imageaware.ImageAware
* image aware view} will be reset (set <b>null</b>) before image loading start
*
* @deprecated Use {@link #resetViewBeforeLoading(boolean) resetViewBeforeLoading(true)} instead
*/
public Builder resetViewBeforeLoading() {
resetViewBeforeLoading = true;
return this;
}
/**
* Sets whether {@link com.nostra13.universalimageloader.core.imageaware.ImageAware
* image aware view} will be reset (set <b>null</b>) before image loading start
*/
public Builder resetViewBeforeLoading(boolean resetViewBeforeLoading) {
this.resetViewBeforeLoading = resetViewBeforeLoading;
return this;
}
/**
* Loaded image will be cached in memory
*
* @deprecated Use {@link #cacheInMemory(boolean) cacheInMemory(true)} instead
*/
@Deprecated
public Builder cacheInMemory() {
cacheInMemory = true;
return this;
}
/** Sets whether loaded image will be cached in memory */
public Builder cacheInMemory(boolean cacheInMemory) {
this.cacheInMemory = cacheInMemory;
return this;
}
/**
* Loaded image will be cached on disk
*
* @deprecated Use {@link #cacheOnDisk(boolean) cacheOnDisk(true)} instead
*/
@Deprecated
public Builder cacheOnDisc() {
return cacheOnDisk(true);
}
/**
* Sets whether loaded image will be cached on disk
*
* @deprecated Use {@link #cacheOnDisk(boolean)} instead
*/
@Deprecated
public Builder cacheOnDisc(boolean cacheOnDisk) {
return cacheOnDisk(cacheOnDisk);
}
/** Sets whether loaded image will be cached on disk */
public Builder cacheOnDisk(boolean cacheOnDisk) {
this.cacheOnDisk = cacheOnDisk;
return this;
}
/**
* Sets {@linkplain ImageScaleType scale type} for decoding image. This parameter is used while define scale
* size for decoding image to Bitmap. Default value - {@link ImageScaleType#IN_SAMPLE_POWER_OF_2}
*/
public Builder imageScaleType(ImageScaleType imageScaleType) {
this.imageScaleType = imageScaleType;
return this;
}
/** Sets {@link Bitmap.Config bitmap config} for image decoding. Default value - {@link Bitmap.Config#ARGB_8888} */
public Builder bitmapConfig(Bitmap.Config bitmapConfig) {
if (bitmapConfig == null) throw new IllegalArgumentException("bitmapConfig can't be null");
decodingOptions.inPreferredConfig = bitmapConfig;
return this;
}
/**
* Sets options for image decoding.<br />
* <b>NOTE:</b> {@link Options#inSampleSize} of incoming options will <b>NOT</b> be considered. Library
* calculate the most appropriate sample size itself according yo {@link #imageScaleType(ImageScaleType)}
* options.<br />
* <b>NOTE:</b> This option overlaps {@link #bitmapConfig(android.graphics.Bitmap.Config) bitmapConfig()}
* option.
*/
public Builder decodingOptions(Options decodingOptions) {
if (decodingOptions == null) throw new IllegalArgumentException("decodingOptions can't be null");
this.decodingOptions = decodingOptions;
return this;
}
/** Sets delay time before starting loading task. Default - no delay. */
public Builder delayBeforeLoading(int delayInMillis) {
this.delayBeforeLoading = delayInMillis;
return this;
}
/** Sets auxiliary object which will be passed to {@link ImageDownloader#getStream(String, Object)} */
public Builder extraForDownloader(Object extra) {
this.extraForDownloader = extra;
return this;
}
/** Sets whether ImageLoader will consider EXIF parameters of JPEG image (rotate, flip) */
public Builder considerExifParams(boolean considerExifParams) {
this.considerExifParams = considerExifParams;
return this;
}
/**
* Sets bitmap processor which will be process bitmaps before they will be cached in memory. So memory cache
* will contain bitmap processed by incoming preProcessor.<br />
* Image will be pre-processed even if caching in memory is disabled.
*/
public Builder preProcessor(BitmapProcessor preProcessor) {
this.preProcessor = preProcessor;
return this;
}
/**
* Sets bitmap processor which will be process bitmaps before they will be displayed in
* {@link com.nostra13.universalimageloader.core.imageaware.ImageAware image aware view} but
* after they'll have been saved in memory cache.
*/
public Builder postProcessor(BitmapProcessor postProcessor) {
this.postProcessor = postProcessor;
return this;
}
/**
* Sets custom {@link BitmapDisplayer displayer} for image loading task. Default value -
* {@link DefaultConfigurationFactory#createBitmapDisplayer()}
*/
public Builder displayer(BitmapDisplayer displayer) {
if (displayer == null) throw new IllegalArgumentException("displayer can't be null");
this.displayer = displayer;
return this;
}
Builder syncLoading(boolean isSyncLoading) {
this.isSyncLoading = isSyncLoading;
return this;
}
/**
* Sets custom {@linkplain Handler handler} for displaying images and firing {@linkplain ImageLoadingListener
* listener} events.
*/
public Builder handler(Handler handler) {
this.handler = handler;
return this;
}
/** Sets all options equal to incoming options */
public Builder cloneFrom(DisplayImageOptions options) {
imageResOnLoading = options.imageResOnLoading;
imageResForEmptyUri = options.imageResForEmptyUri;
imageResOnFail = options.imageResOnFail;
imageOnLoading = options.imageOnLoading;
imageForEmptyUri = options.imageForEmptyUri;
imageOnFail = options.imageOnFail;
resetViewBeforeLoading = options.resetViewBeforeLoading;
cacheInMemory = options.cacheInMemory;
cacheOnDisk = options.cacheOnDisk;
imageScaleType = options.imageScaleType;
decodingOptions = options.decodingOptions;
delayBeforeLoading = options.delayBeforeLoading;
considerExifParams = options.considerExifParams;
extraForDownloader = options.extraForDownloader;
preProcessor = options.preProcessor;
postProcessor = options.postProcessor;
displayer = options.displayer;
handler = options.handler;
isSyncLoading = options.isSyncLoading;
return this;
}
/** Builds configured {@link DisplayImageOptions} object */
public DisplayImageOptions build() {
return new DisplayImageOptions(this);
}
}
/**
* Creates options appropriate for single displaying:
* <ul>
* <li>View will <b>not</b> be reset before loading</li>
* <li>Loaded image will <b>not</b> be cached in memory</li>
* <li>Loaded image will <b>not</b> be cached on disk</li>
* <li>{@link ImageScaleType#IN_SAMPLE_POWER_OF_2} decoding type will be used</li>
* <li>{@link Bitmap.Config#ARGB_8888} bitmap config will be used for image decoding</li>
* <li>{@link SimpleBitmapDisplayer} will be used for image displaying</li>
* </ul>
* <p/>
* These option are appropriate for simple single-use image (from drawables or from Internet) displaying.
*/
public static DisplayImageOptions createSimple() {
return new Builder().build();
}
}
虽然这个有点长，但是结构还是很清新明了的。
首先我们可以看到我们需要一个 DisplayImageOptions 的对象类，但是这个类需要配置的信息众多（63-75行），如果全都由构造方法传入的话显然不可能，这里使用了建造者模式来完成 DisplayImageOptions 对象类的构建。
这里的建造者是一个静态内部类 DisplayImageOptions.Builder，担任了建造者模式中抽象建造者和具体建造者两个职责。由 build() 方法返回构建的 DisplayImageOptions 实例，其余方法负责具体组件的配置并返回 Builder 对象，于是我们可以像下面这样调用：
DisplayImageOptions options = new DisplayImageOptions.Builder()
.cacheInMemory(true) // 设置下载的图片是否缓存在内存中
.cacheOnDisk(true) // 设置下载的图片是否缓存在本地中
.showImageForEmptyUri(R.mipmap.icon_head) // 空的Uri时显示的图片
.showImageOnFail(R.mipmap.icon_head) // 错误时显示的图片
.showImageOnLoading(R.mipmap.icon_head) // 加载时显示的图片
.build();
这样使用起来是不是十分简单和清晰了呢。
查看更多：设计模式分类以及六大设计原则