ServletContext对象常用操作

WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext方法获得ServletContext对象。由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。ServletContext对象通常也被称之为context域对象。

操作一：获取全局配置参数

有两种方式获取ServletContext对象，第一种是通过ServletConfig对象获取

super.getServletConfig().getServletContext();

第二种方式是直接获取，GenericServlet中提供了这个方法，实际上也是调用哪个了ServletConfig

 @Override
    public ServletContext getServletContext() {
        return getServletConfig().getServletContext();
    }

通过ServletContext获取全局配置参数

public class ParamContext extends HttpServlet {
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		String param = getServletContext().getInitParameter("param");
		System.out.println(param);
	}
}

web.xml中的配置，这种全局参数所有Servlet都可以访问到

<context-param>
		<param-name>param</param-name>
		<param-value>I'm param</param-value>
	</context-param>

操作二：获取文件的MIME类型

Serlvet会读取Tomcat/conf/web.xml中所配置MIME类型，文件下载时会用到

public class MIMEContext extends HttpServlet {
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		String doc = super.getServletContext().getMimeType("file.doc");
		String jpg = super.getServletContext().getMimeType("file.jpg");
		String txt = super.getServletContext().getMimeType("file.txt");
		System.out.println(doc + "\t" + jpg + "\t" + txt);
	}
}

操作三：日志信息

Serlvet容器启动时会在console中输出大量信息，通过SerlvetContext也可以添加自己的日志信息

public class ParamContext extends HttpServlet {
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		super.getServletConfig().getServletContext();
		String param = getServletContext().getInitParameter("param");
		System.out.println(param);
	}

操作四：Servlet间共享数据

SerlvetContext对象本生代表着属性范围，可以通过Servlet对象调用Attribute设置参数的方式，在多个Servlet间共享数据

编写一个Servlet统计访问该Servlet的次数

public class VisitContext extends HttpServlet {

	@Override
	public void init() throws ServletException {
		super.getServletContext().setAttribute("visit", 0);
	}

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		ServletContext context = super.getServletContext();
		Integer count = (Integer) context.getAttribute("visit");
		count++;
		context.setAttribute("visit", count);
	}
}

在另外一个Servlet中获取，该访问次数

public class ShareContext extends HttpServlet {
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		Integer count = (Integer) super.getServletContext().getAttribute("visit");
		System.out.println(count);
	}
}

操作五：Web工程中获取绝对路径

在Servlet中读取文件必须使用磁盘的绝对路径，在src下建议个文件，使用Serlvet获取该路径

public class ResourceContext extends HttpServlet {
	protected void doGet(javax.servlet.http.HttpServletRequest req, javax.servlet.http.HttpServletResponse resp) throws javax.servlet.ServletException, java.io.IOException {
		String file = "/WEB-INF/classes/flie1.txt";
		String realPath = super.getServletContext().getRealPath(file);
		System.out.println(realPath);
	};
}

如果只是获取classpath下的文件，可以使用类加载器，Hibernate框架加载文件就是用这种方法，和ServletContext完全解耦，getReal()、getFile()一样

public class ResourceContext extends HttpServlet {
	protected void doGet(javax.servlet.http.HttpServletRequest req, javax.servlet.http.HttpServletResponse resp) throws javax.servlet.ServletException, java.io.IOException {
		String file = "/WEB-INF/classes/file1.txt";//src下
		System.out.println(ResourceContext.class.getResource(file));
		System.out.println(ResourceContext.class.getResource(file).getPath());
		System.out.println(ResourceContext.class.getResource(file).getFile());
	}
}

工程下的文件无法获取，WebRoot以外的文件不会发布到Tomcat中

getServletContext().getRealPath("/2.txt");//WebRoot下
getServletContext().getRealPath("/WEB-INF/3.txt");//WEB-INF下
getServletContext().getRealPath("/WEB-INF/classes/4.txt");//src下

操作六：读取properties文件

public class ServletTest2 extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		
		InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/you/servlet/db.properties");
        Properties pro = new Properties();//Map形式
        pro.load(in);
        
        String url = pro.getProperty("url");
        String username = pro.getProperty("username");
        String password = pro.getProperty("password");
        
        System.out.println(url + " " + username + " " + password);
	}
}

ServletContext对象常用操作

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