看高手代碼--從小case學大道理

今天看sun的HttpMessages.java文件，雖然文件很小，但是對我的啓發很大。
前面定義了大量的HTTP常量：

...
...
private static final String STATUS_305 = "Use Proxy";
private static final String STATUS_307 = "Temporary Redirect";
private static final String STATUS_400 = "Bad Request";
private static final String STATUS_401 = "Unauthorized";
private static final String STATUS_402 = "Payment Required";
private static final String STATUS_403 = "Forbidden";
private static final String STATUS_404 = "Not Found";
private static final String STATUS_405 = "Method Not Allowed";
private static final String STATUS_406 = "Not Acceptable";
private static final String STATUS_407 = "Proxy Authentication Required";
private static final String STATUS_408 = "Request Timeout";
...
...

    ...
    ...
    private static final String STATUS_305 = "Use Proxy";
    private static final String STATUS_307 = "Temporary Redirect";
    private static final String STATUS_400 = "Bad Request";
    private static final String STATUS_401 = "Unauthorized";
    private static final String STATUS_402 = "Payment Required";
    private static final String STATUS_403 = "Forbidden";
    private static final String STATUS_404 = "Not Found";
    private static final String STATUS_405 = "Method Not Allowed";
    private static final String STATUS_406 = "Not Acceptable";
    private static final String STATUS_407 = "Proxy Authentication Required";
    private static final String STATUS_408 = "Request Timeout";
    ...
    ...

也可以趁機從這裏查看HTTP返回值的意義^_^。然後定義了一個java.util.concurrent.ConcurrentHashMap的變量httpStatusCodeMappings，再往裏面添加剛纔定義的常量（這個操作放在了一個靜態塊裏），如：

static {
        httpStatusCodeMappings.put("sc.100", STATUS_100);
        httpStatusCodeMappings.put("sc.101", STATUS_101);
        httpStatusCodeMappings.put("sc.200", STATUS_200);
        httpStatusCodeMappings.put("sc.201", STATUS_201);
        httpStatusCodeMappings.put("sc.202", STATUS_202);
        httpStatusCodeMappings.put("sc.203", STATUS_203);
  ...
  ...

static {
        httpStatusCodeMappings.put("sc.100", STATUS_100);
        httpStatusCodeMappings.put("sc.101", STATUS_101);
        httpStatusCodeMappings.put("sc.200", STATUS_200);
        httpStatusCodeMappings.put("sc.201", STATUS_201);
        httpStatusCodeMappings.put("sc.202", STATUS_202);
        httpStatusCodeMappings.put("sc.203", STATUS_203);
  ...
  ...

最重要的就是這個方法了，先看代碼：

public static String getMessage(int status) {

        // Return the status message for the most frequently used status
        // codes directly, without any lookup
        switch (status) {
            case 200: return STATUS_200;
            case 302: return STATUS_302;
            case 400: return STATUS_400;
            case 404: return STATUS_404;
    }

        return httpStatusCodeMappings.get("sc."+ status);
    }

public static String getMessage(int status) {

        // Return the status message for the most frequently used status
        // codes directly, without any lookup
        switch (status) {
            case 200: return STATUS_200;
            case 302: return STATUS_302;
            case 400: return STATUS_400;
            case 404: return STATUS_404;
	}

        return httpStatusCodeMappings.get("sc."+ status);
    }

這段代碼的用途很簡單，就是要返回狀態碼對應的Message，而這些這些消息都已經存放在httpStatusCodeMappings變量裏了，爲了提高訪問常用的幾個返回碼的Message，它直接用了一個靜態塊，而不去lookup那個map了，這就是高手！