update readme file with example for http servlet instrumentation. (#1672)

rghetia · web-flow · commit 79e16442ac1e · 2019-01-17T12:28:10.000-08:00
diff --git a/contrib/http_servlet/README.md b/contrib/http_servlet/README.md
@@ -31,6 +31,100 @@ compile 'io.opencensus:opencensus-api:0.19.0'
 compile 'io.opencensus:opencensus-contrib-http-servlet:0.19.0'
 ```
 
+## Instrumenting HTTP Servlets
+
+Users can instrument Http Servlets by simply adding OcHttpServletFilter.class to your servlet filter.
+
+Below is an example of how a servlet is instrumented.
+
+```java
+
+import io.opencensus.contrib.http.servlet.OcHttpServletFilter;
+import io.opencensus.contrib.http.util.HttpViews;
+import io.opencensus.exporter.stats.prometheus.PrometheusStatsCollector;
+import io.opencensus.exporter.trace.logging.LoggingTraceExporter;
+import io.prometheus.client.exporter.HTTPServer;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.util.EnumSet;
+import javax.servlet.DispatcherType;
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import org.eclipse.jetty.server.Request;
+import org.eclipse.jetty.server.Server;
+import org.eclipse.jetty.server.handler.AbstractHandler;
+import org.eclipse.jetty.servlet.ServletContextHandler;
+import org.eclipse.jetty.servlet.ServletHandler;
+
+public class HelloWorldServer extends AbstractHandler {
+
+  public static class HelloServlet extends HttpServlet {
+    private static final long serialVersionUID = 1L;
+
+    @Override
+    protected void doGet(HttpServletRequest request, HttpServletResponse response)
+        throws ServletException, IOException {
+      PrintWriter pout = response.getWriter();
+
+      pout.print("<html><body><h3>Hello Servlet Get</h3></body></html>");
+      return;
+    }
+
+    @Override
+    protected void doPost(HttpServletRequest request, HttpServletResponse response)
+        throws ServletException, IOException {
+
+      PrintWriter pout = response.getWriter();
+      pout.print("<html><body><h3>Hello Servlet Post</h3></body></html>");
+      return;
+    }
+  }
+
+  public void handle(
+      String target, Request baseRequest, HttpServletRequest request, HttpServletResponse response)
+      throws IOException, ServletException {
+    response.setContentType("text/html;charset=utf-8");
+    response.setStatus(HttpServletResponse.SC_OK);
+    baseRequest.setHandled(true);
+    response.getWriter().println("<h1>Hello World. default handle</h1>");
+  }
+
+  private static void initStatsExporter() throws IOException {
+    HttpViews.registerAllServerViews();
+
+    // Register stats exporter to a desired backend.
+    PrometheusStatsCollector.createAndRegister();
+    HTTPServer prometheusServer = new HTTPServer(9090, true);
+  }
+
+  private static void initTracing() {
+    // Register trace exporter to a desired backend.
+    LoggingTraceExporter.register();
+  }
+
+  public static void main(String[] args) throws Exception {
+    initTracing();
+    initStatsExporter();
+    ServletContextHandler context = new ServletContextHandler(ServletContextHandler.SESSIONS);
+    context.setContextPath("/");
+
+    Server server = new Server(8080);
+    ServletHandler handler = new ServletHandler();
+    server.setHandler(handler);
+
+    // Add OcHttpServletFilter to enable tracing and monitoring stats.
+    handler.addFilterWithMapping(
+        OcHttpServletFilter.class, "/*", EnumSet.of(DispatcherType.REQUEST));
+    handler.addServletWithMapping(HelloServlet.class, "/*");
+
+    server.start();
+    server.join();
+  }
+}
+```
+
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