设计模式复习--简单工厂模式

   模式场景：在一个披萨店中，要根据不同客户的口味，生产不同的披萨，如素食披萨、希腊披萨等披萨。

        该例的UML结构图如下:

 Pizza制造工厂：SimplyPizzaFactory.java

/**
 * 专门用于创建披萨的工厂类
 */
public class SimplePizzaFactory {
        public Pizza createPizza(String type){
                Pizza pizza = null;
                
                if(type.equals("cheese")){
                        pizza = new CheesePizza();
                }
                else if(type.equals("clam")){
                        pizza = new ClamPizza();
                }
                else if(type.equals("pepperoni")){
                        pizza = new PepperoniPizza();
                }
                else if(type.equals("veggie")){
                        pizza = new VeggiePizze();
                }
                
                return pizza;
        }
}

其中CheesePizza，ClamPizza，PepperoniPizza，VeggiePizze是集成自Pizza的子类

Pizza.java

/**
 * 抽象pizza类
 */
public abstract class Pizza {
        public abstract void prepare();
        
        public abstract void bake();
        
        public abstract void cut();
        
        public abstract void box();
}

CheesePizza.java

public class CheesePizza extends Pizza{

        @Override
        public void bake() {
                System.out.println("bake CheesePizza ...");
        }

        @Override
        public void box() {
                System.out.println("box CheesePizza ...");
        }

        @Override
        public void cut() {
                System.out.println("cut CheesePizza ...");
        }

        @Override
        public void prepare() {
                System.out.println("prepare CheesePizza ...");
        }
}

PizzaStore.java

public class PizzaStore {
        SimplePizzaFactory factory;      //SimplePizzaFactory的引用
        public PizzaStore(SimplePizzaFactory factory){
                this.factory = factory;
        }
        
        public Pizza orderPizza(String type){
            Pizza pizza;
            pizza = factory.createPizza(type);     //使用工厂对象的创建方法，而不是直接new。这里不再使用具体实例化
                
            pizza.prepare();
            pizza.bake();
            pizza.cut();
            pizza.box();
                
            return pizza;
        }
}

优缺点 

   　　优点

            1、简单工厂模式实现了对责任的分割，提供了专门的工厂类用于创建对象。

            2、客户端无须知道所创建的具体产品类的类名，只需要知道具体产品类所对应的参数即可，对于一些复杂的类名，通过简单工厂模式可以减少使用者的记忆量。

            3、通过引入配置文件，可以在不修改任何客户端代码的情况下更换和增加新的具体产品类，在一定程度上提高了系统的灵活性。

 

         缺点

            1、由于工厂类集中了所有产品创建逻辑，一旦不能正常工作，整个系统都要受到影响。

            2、使用简单工厂模式将会增加系统中类的个数，在一定程序上增加了系统的复杂度和理解难度。

            3、系统扩展困难，一旦添加新产品就不得不修改工厂逻辑，在产品类型较多时，有可能造成工厂逻辑过于复杂，不利于系统的扩展和维护。

            4、简单工厂模式由于使用了静态工厂方法，造成工厂角色无法形成基于继承的等级结构。

　　　　5、 简单工厂模式最大的优点在于实现对象的创建和对象的使用分离，但是如果产品过多时，会导致工厂代码非常复杂。

android源码里面使用工厂设计模式的地方：

BitMapFactory，还有就是Volley里面对HttpStack的实例化代码：

HttpStack为网络模块的接口；

public interface HttpStack {
    /**
     * Performs an HTTP request with the given parameters.
     *
     * <p>A GET request is sent if request.getPostBody() == null. A POST request is sent otherwise,
     * and the Content-Type header is set to request.getPostBodyContentType().</p>
     *
     * @param request the request to perform
     * @param additionalHeaders additional headers to be sent together with
     *         {@link Request#getHeaders()}
     * @return the HTTP response
     */
    public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
        throws IOException, AuthFailureError;

}

HurlStack为实现httpStack的类，用httpUrlConnection来实现网络操作；

public class HurlStack implements HttpStack {

    private static final String HEADER_CONTENT_TYPE = "Content-Type";

    /**
     * An interface for transforming URLs before use.
     */
    public interface UrlRewriter {
        /**
         * Returns a URL to use instead of the provided one, or null to indicate
         * this URL should not be used at all.
         */
        public String rewriteUrl(String originalUrl);
    }

    private final UrlRewriter mUrlRewriter;
    private final SSLSocketFactory mSslSocketFactory;

    public HurlStack() {
        this(null);
    }

    /**
     * @param urlRewriter Rewriter to use for request URLs
     */
    public HurlStack(UrlRewriter urlRewriter) {
        this(urlRewriter, null);
    }

    /**
     * @param urlRewriter Rewriter to use for request URLs
     * @param sslSocketFactory SSL factory to use for HTTPS connections
     */
    public HurlStack(UrlRewriter urlRewriter, SSLSocketFactory sslSocketFactory) {
        mUrlRewriter = urlRewriter;
        mSslSocketFactory = sslSocketFactory;
    }

    @Override
    public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
            throws IOException, AuthFailureError {
        String url = request.getUrl();
        HashMap<String, String> map = new HashMap<String, String>();
        map.putAll(request.getHeaders());
        map.putAll(additionalHeaders);
        if (mUrlRewriter != null) {
            String rewritten = mUrlRewriter.rewriteUrl(url);
            if (rewritten == null) {
                throw new IOException("URL blocked by rewriter: " + url);
            }
            url = rewritten;
        }
        URL parsedUrl = new URL(url);
        HttpURLConnection connection = openConnection(parsedUrl, request);
        for (String headerName : map.keySet()) {
            connection.addRequestProperty(headerName, map.get(headerName));
        }
        setConnectionParametersForRequest(connection, request);
        // Initialize HttpResponse with data from the HttpURLConnection.
        ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);
        int responseCode = connection.getResponseCode();
        if (responseCode == -1) {
            // -1 is returned by getResponseCode() if the response code could not be retrieved.
            // Signal to the caller that something was wrong with the connection.
            throw new IOException("Could not retrieve response code from HttpUrlConnection.");
        }
        StatusLine responseStatus = new BasicStatusLine(protocolVersion,
                connection.getResponseCode(), connection.getResponseMessage());
        BasicHttpResponse response = new BasicHttpResponse(responseStatus);
        response.setEntity(entityFromConnection(connection));
        for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {
            if (header.getKey() != null) {
                Header h = new BasicHeader(header.getKey(), header.getValue().get(0));
                response.addHeader(h);
            }
        }
        return response;
    }

    /**
     * Initializes an {@link HttpEntity} from the given {@link HttpURLConnection}.
     * @param connection
     * @return an HttpEntity populated with data from <code>connection</code>.
     */
    private static HttpEntity entityFromConnection(HttpURLConnection connection) {
        BasicHttpEntity entity = new BasicHttpEntity();
        InputStream inputStream;
        try {
            inputStream = connection.getInputStream();
        } catch (IOException ioe) {
            inputStream = connection.getErrorStream();
        }
        entity.setContent(inputStream);
        entity.setContentLength(connection.getContentLength());
        entity.setContentEncoding(connection.getContentEncoding());
        entity.setContentType(connection.getContentType());
        return entity;
    }

    /**
     * Create an {@link HttpURLConnection} for the specified {@code url}.
     */
    protected HttpURLConnection createConnection(URL url) throws IOException {
        return (HttpURLConnection) url.openConnection();
    }

    /**
     * Opens an {@link HttpURLConnection} with parameters.
     * @param url
     * @return an open connection
     * @throws IOException
     */
    private HttpURLConnection openConnection(URL url, Request<?> request) throws IOException {
        HttpURLConnection connection = createConnection(url);

        int timeoutMs = request.getTimeoutMs();
        connection.setConnectTimeout(timeoutMs);
        connection.setReadTimeout(timeoutMs);
        connection.setUseCaches(false);
        connection.setDoInput(true);

        // use caller-provided custom SslSocketFactory, if any, for HTTPS
        if ("https".equals(url.getProtocol()) && mSslSocketFactory != null) {
            ((HttpsURLConnection)connection).setSSLSocketFactory(mSslSocketFactory);
        }

        return connection;
    }

    @SuppressWarnings("deprecation")
    /* package */ static void setConnectionParametersForRequest(HttpURLConnection connection,
            Request<?> request) throws IOException, AuthFailureError {
        switch (request.getMethod()) {
            case Method.DEPRECATED_GET_OR_POST:
                // This is the deprecated way that needs to be handled for backwards compatibility.
                // If the request's post body is null, then the assumption is that the request is
                // GET.  Otherwise, it is assumed that the request is a POST.
                byte[] postBody = request.getPostBody();
                if (postBody != null) {
                    // Prepare output. There is no need to set Content-Length explicitly,
                    // since this is handled by HttpURLConnection using the size of the prepared
                    // output stream.
                    connection.setDoOutput(true);
                    connection.setRequestMethod("POST");
                    connection.addRequestProperty(HEADER_CONTENT_TYPE,
                            request.getPostBodyContentType());
                    DataOutputStream out = new DataOutputStream(connection.getOutputStream());
                    out.write(postBody);
                    out.close();
                }
                break;
            case Method.GET:
                // Not necessary to set the request method because connection defaults to GET but
                // being explicit here.
                connection.setRequestMethod("GET");
                break;
            case Method.DELETE:
                connection.setRequestMethod("DELETE");
                break;
            case Method.POST:
                connection.setRequestMethod("POST");
                addBodyIfExists(connection, request);
                break;
            case Method.PUT:
                connection.setRequestMethod("PUT");
                addBodyIfExists(connection, request);
                break;
            case Method.HEAD:
                connection.setRequestMethod("HEAD");
                break;
            case Method.OPTIONS:
                connection.setRequestMethod("OPTIONS");
                break;
            case Method.TRACE:
                connection.setRequestMethod("TRACE");
                break;
            case Method.PATCH:
                connection.setRequestMethod("PATCH");
                addBodyIfExists(connection, request);
                break;
            default:
                throw new IllegalStateException("Unknown method type.");
        }
    }

    private static void addBodyIfExists(HttpURLConnection connection, Request<?> request)
            throws IOException, AuthFailureError {
        byte[] body = request.getBody();
        if (body != null) {
            connection.setDoOutput(true);
            connection.addRequestProperty(HEADER_CONTENT_TYPE, request.getBodyContentType());
            DataOutputStream out = new DataOutputStream(connection.getOutputStream());
            out.write(body);
            out.close();
        }
    }
}

HttpClientStack也是实现HttpStack的类，用HttpClient来实现网络操作；

public class HttpClientStack implements HttpStack {
    protected final HttpClient mClient;

    private final static String HEADER_CONTENT_TYPE = "Content-Type";

    public HttpClientStack(HttpClient client) {
        mClient = client;
    }

    private static void addHeaders(HttpUriRequest httpRequest, Map<String, String> headers) {
        for (String key : headers.keySet()) {
            httpRequest.setHeader(key, headers.get(key));
        }
    }

    @SuppressWarnings("unused")
    private static List<NameValuePair> getPostParameterPairs(Map<String, String> postParams) {
        List<NameValuePair> result = new ArrayList<NameValuePair>(postParams.size());
        for (String key : postParams.keySet()) {
            result.add(new BasicNameValuePair(key, postParams.get(key)));
        }
        return result;
    }

    @Override
    public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
            throws IOException, AuthFailureError {
        HttpUriRequest httpRequest = createHttpRequest(request, additionalHeaders);
        addHeaders(httpRequest, additionalHeaders);
        addHeaders(httpRequest, request.getHeaders());
        onPrepareRequest(httpRequest);
        HttpParams httpParams = httpRequest.getParams();
        int timeoutMs = request.getTimeoutMs();
        // TODO: Reevaluate this connection timeout based on more wide-scale
        // data collection and possibly different for wifi vs. 3G.
        HttpConnectionParams.setConnectionTimeout(httpParams, 5000);
        HttpConnectionParams.setSoTimeout(httpParams, timeoutMs);
        return mClient.execute(httpRequest);
    }

    /**
     * Creates the appropriate subclass of HttpUriRequest for passed in request.
     */
    @SuppressWarnings("deprecation")
    /* protected */ static HttpUriRequest createHttpRequest(Request<?> request,
            Map<String, String> additionalHeaders) throws AuthFailureError {
        switch (request.getMethod()) {
            case Method.DEPRECATED_GET_OR_POST: {
                // This is the deprecated way that needs to be handled for backwards compatibility.
                // If the request's post body is null, then the assumption is that the request is
                // GET.  Otherwise, it is assumed that the request is a POST.
                byte[] postBody = request.getPostBody();
                if (postBody != null) {
                    HttpPost postRequest = new HttpPost(request.getUrl());
                    postRequest.addHeader(HEADER_CONTENT_TYPE, request.getPostBodyContentType());
                    HttpEntity entity;
                    entity = new ByteArrayEntity(postBody);
                    postRequest.setEntity(entity);
                    return postRequest;
                } else {
                    return new HttpGet(request.getUrl());
                }
            }
            case Method.GET:
                return new HttpGet(request.getUrl());
            case Method.DELETE:
                return new HttpDelete(request.getUrl());
            case Method.POST: {
                HttpPost postRequest = new HttpPost(request.getUrl());
                postRequest.addHeader(HEADER_CONTENT_TYPE, request.getBodyContentType());
                setEntityIfNonEmptyBody(postRequest, request);
                return postRequest;
            }
            case Method.PUT: {
                HttpPut putRequest = new HttpPut(request.getUrl());
                putRequest.addHeader(HEADER_CONTENT_TYPE, request.getBodyContentType());
                setEntityIfNonEmptyBody(putRequest, request);
                return putRequest;
            }
            case Method.HEAD:
                return new HttpHead(request.getUrl());
            case Method.OPTIONS:
                return new HttpOptions(request.getUrl());
            case Method.TRACE:
                return new HttpTrace(request.getUrl());
            case Method.PATCH: {
                HttpPatch patchRequest = new HttpPatch(request.getUrl());
                patchRequest.addHeader(HEADER_CONTENT_TYPE, request.getBodyContentType());
                setEntityIfNonEmptyBody(patchRequest, request);
                return patchRequest;
            }
            default:
                throw new IllegalStateException("Unknown request method.");
        }
    }

    private static void setEntityIfNonEmptyBody(HttpEntityEnclosingRequestBase httpRequest,
            Request<?> request) throws AuthFailureError {
        byte[] body = request.getBody();
        if (body != null) {
            HttpEntity entity = new ByteArrayEntity(body);
            httpRequest.setEntity(entity);
        }
    }

    /**
     * Called before the request is executed using the underlying HttpClient.
     *
     * <p>Overwrite in subclasses to augment the request.</p>
     */
    protected void onPrepareRequest(HttpUriRequest request) throws IOException {
        // Nothing.
    }

    /**
     * The HttpPatch class does not exist in the Android framework, so this has been defined here.
     */
    public static final class HttpPatch extends HttpEntityEnclosingRequestBase {

        public final static String METHOD_NAME = "PATCH";

        public HttpPatch() {
            super();
        }

        public HttpPatch(final URI uri) {
            super();
            setURI(uri);
        }

        /**
         * @throws IllegalArgumentException if the uri is invalid.
         */
        public HttpPatch(final String uri) {
            super();
            setURI(URI.create(uri));
        }

        @Override
        public String getMethod() {
            return METHOD_NAME;
        }

    }
}

两个都是为了实现HttpStack里面的performRequest方法；

然后在Volley的类中newRequestQueue里面根据不同的android 的版本号，选择使用HttpUrlConnection的HurlStack或者使用HttpClientStack的HurlStack；

    /**
     * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
     *获取Volley对象
     * @param context A {@link Context} to use for creating the cache dir.
     * @param stack An {@link HttpStack} to use for the network, or null for default.
     * @return A started {@link RequestQueue} instance.
     */
    @SuppressLint("NewApi")
    public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
        File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);

        String userAgent = "volley/0";
        try {
            String packageName = context.getPackageName();
            PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);
            userAgent = packageName + "/" + info.versionCode;
        } catch (NameNotFoundException e) {
        }

        if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                stack = new HurlStack();
            } else {
                // Prior to Gingerbread, HttpUrlConnection was unreliable.
                // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }

        Network network = new BasicNetwork(stack);

        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();

        return queue;
        /*
         * 实例化一个RequestQueue，其中start()主要完成相关工作线程的开启，
         * 比如开启缓存线程CacheDispatcher先完成缓存文件的扫描， 还包括开启多个NetworkDispatcher访问网络线程，
         * 该多个网络线程将从 同一个 网络阻塞队列中读取消息
         * 
         * 此处可见，start()已经开启，所有我们不用手动的去调用该方法，在start()方法中如果存在工作线程应该首先终止，并重新实例化工作线程并开启
         * 在访问网络很频繁，而又重复调用start()，势必会导致性能的消耗；但是如果在访问网络很少时，调用stop()方法，停止多个线程，然后调用start(),反而又可以提高性能，具体可折中选择
         */
    }

---

2015年12月8日20:37:57更新

android中工厂设计模式的使用：

比如一个业务数据的来源有三种，分别是数据库，xml文件，文件，我们就可以按照如下方式来实现：

1，创建一个基类抽象类，抽象对数据库，xml,文件的增删改查的方法。

2，继承抽象基类，并实现基类的抽象方法（数据库实现类，xml实现类，文件实现类）。

3，然后就是创建如下的简单工厂类来实现：

// 抽象的工厂类，定义了其子类必须实现的createProduct()方法 
  abstract class Factory { 
    //运用了Java 中的泛型和反射技术 
    public abstract <T extends IOHandler> T createProduct(Class<T> c); 
  } 
    
  class ConcreteFactory extends Factory { 
    public <T extends IOHandler> T createProduct(Class<T> c) { 
      T handler = null; 
      try { 
        handler = (T) Class.forName(c.getName()).newInstance(); 
      } catch (Exception e) { 
        e.printStackTrace(); 
      } 
      return handler; 
    } 
  } 

使用数据库，就传进去数据库的class,调用数据库相关操作；

使用xml,传进去xml的class,调用xml的相关操作；

使用文件，闯进去文件操作的class，调用使用文件的相关操作。

使用简单工厂的缺点就是每次我们写新得产品的时候，总是创建一个新类，引入抽象类，到时类数量增加。