第七章 适配器模式 Adapter

结构型设计模式

定义：

　　将一个类的接口，转换成客户期望的另一个接口。适配器让原本接口不兼容的类可以合作无间

类图：

　　

 

 应用：

　　SpringMVC中的适配器模式，在SpringMVC中，使用DispatcherSevlet拦截所有请求，再根据不同的uri调用对应的服务方法。DispatcherSevlet中最核心的doDispatch()方法，截取一段分发服务的代码，这里就用到了适配器模式

　　

    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    ...
        try {
            ModelAndView mv = null;
            Exception dispatchException = null;

            try {
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler.
                String method = request.getMethod();
                boolean isGet = HttpMethod.GET.matches(method);
                if (isGet || HttpMethod.HEAD.matches(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }

                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                // Actually invoke the handler.
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
    ...
    }

　　首先我们来看这个适配器接口（HandlerAdapter ）

public interface HandlerAdapter {
    boolean supports(Object var1);

    @Nullable
    ModelAndView handle(HttpServletRequest var1, HttpServletResponse var2, Object var3) throws Exception;

    long getLastModified(HttpServletRequest var1, Object var2);
}

　　HandlerAdapter接口约束了他的子类必须实现的三个方法，判断是否支持这个handler，执行handler并返回ModelAndView对象，获取最近一次修改时间。很明显这个HandlerAdapter就是为了针对不同的服务实现返回同样的视图。这里，需要了解一下SpringMVC中有多少种服务的实现方式，看一下下面的实现类：

　　最典型的三个：

1. 使用@RequestMapping修饰的方法。这里就是使用RequestMappingHandlerAdapter，去处理这个服务请求，想知道细节的可以去源码看
   
2. 实现了Servlet接口。这里就是使用SimpleServletHandlerAdapter

public class SimpleServletHandlerAdapter implements HandlerAdapter {

    @Override
    public boolean supports(Object handler) {
        return (handler instanceof Servlet);
    }

    @Override
    @Nullable
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {

        ((Servlet) handler).service(request, response);
        return null;
    }

    @Override
    public long getLastModified(HttpServletRequest request, Object handler) {
        return -1;
    }

}

3. 实现了Controller接口
   这里就是使用SimpleControllerHandlerAdapter

public class SimpleControllerHandlerAdapter implements HandlerAdapter {

    @Override
    public boolean supports(Object handler) {
        return (handler instanceof Controller);
    }

    @Override
    @Nullable
    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
            throws Exception {

        return ((Controller) handler).handleRequest(request, response);
    }

    @Override
    public long getLastModified(HttpServletRequest request, Object handler) {
        if (handler instanceof LastModified) {
            return ((LastModified) handler).getLastModified(request);
        }
        return -1L;
    }

}

 

Java I/O中适配器模式：

InputStreamReader/OutputStreamWriter