Add a javax.servlet-api exporter (#918)

* Add prometheus-metrics-exporter-servlet-javax

Signed-off-by: Kinshuk Bairagi <[email protected]>

* Add comments for ci to pass

Signed-off-by: Kinshuk Bairagi <[email protected]>

---------

Signed-off-by: Kinshuk Bairagi <[email protected]>

pom.xml

@@ -59,6 +59,7 @@
         <module>prometheus-metrics-exposition-formats</module>
         <module>prometheus-metrics-exporter-common</module>
         <module>prometheus-metrics-exporter-servlet-jakarta</module>
+        <module>prometheus-metrics-exporter-servlet-javax</module>
         <module>prometheus-metrics-exporter-httpserver</module>
         <module>prometheus-metrics-exporter-opentelemetry</module>
         <module>prometheus-metrics-instrumentation-jvm</module>

prometheus-metrics-exporter-servlet-javax/pom.xml

@@ -0,0 +1,53 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <parent>
+        <groupId>io.prometheus</groupId>
+        <artifactId>client_java</artifactId>
+        <version>1.2.0-SNAPSHOT</version>
+    </parent>
+
+    <artifactId>prometheus-metrics-exporter-servlet-javax</artifactId>
+    <packaging>bundle</packaging>
+
+    <name>Prometheus Metrics Exporter - Servlet JavaX</name>
+    <description>
+        JavaX Servlet for exposing a Prometheus scrape endpoint.
+    </description>
+
+    <properties>
+        <automatic.module.name>io.prometheus.metrics.exporter.servlet.javax</automatic.module.name>
+    </properties>
+
+    <licenses>
+        <license>
+            <name>The Apache Software License, Version 2.0</name>
+            <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
+            <distribution>repo</distribution>
+        </license>
+    </licenses>
+
+    <developers>
+        <developer>
+            <id>kingster</id>
+            <name>Kinshuk Bairagi</name>
+            <email>[email protected]</email>
+        </developer>
+    </developers>
+
+    <dependencies>
+        <dependency>
+            <groupId>io.prometheus</groupId>
+            <artifactId>prometheus-metrics-exporter-common</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>javax.servlet</groupId>
+            <artifactId>javax.servlet-api</artifactId>
+            <version>3.1.0</version>
+            <scope>provided</scope>
+        </dependency>
+    </dependencies>
+
+</project>

prometheus-metrics-exporter-servlet-javax/src/main/java/io/prometheus/metrics/exporter/servlet/javax/HttpExchangeAdapter.java

@@ -0,0 +1,153 @@
+package io.prometheus.metrics.exporter.servlet.javax;
+
+import io.prometheus.metrics.exporter.common.PrometheusHttpExchange;
+import io.prometheus.metrics.exporter.common.PrometheusHttpRequest;
+import io.prometheus.metrics.exporter.common.PrometheusHttpResponse;
+
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.util.Enumeration;
+
+/**
+ * This class is an adapter for HTTP exchanges, implementing the PrometheusHttpExchange interface.
+ * It wraps HttpServletRequest and HttpServletResponse objects into Request and Response inner classes.
+ */
+public class HttpExchangeAdapter implements PrometheusHttpExchange {
+
+    private final Request request;
+    private final Response response;
+
+    /**
+     * Constructs a new HttpExchangeAdapter with the given HttpServletRequest and HttpServletResponse.
+     *
+     * @param request  the HttpServletRequest to be adapted
+     * @param response the HttpServletResponse to be adapted
+     */
+    public HttpExchangeAdapter(HttpServletRequest request, HttpServletResponse response) {
+        this.request = new Request(request);
+        this.response = new Response(response);
+    }
+
+    /**
+     * Returns the adapted HttpServletRequest.
+     *
+     * @return the adapted HttpServletRequest
+     */
+    @Override
+    public PrometheusHttpRequest getRequest() {
+        return request;
+    }
+
+    /**
+     * Returns the adapted HttpServletResponse.
+     *
+     * @return the adapted HttpServletResponse
+     */
+    @Override
+    public PrometheusHttpResponse getResponse() {
+        return response;
+    }
+
+    @Override
+    public void handleException(IOException e) throws IOException {
+        throw e; // leave exception handling to the servlet container
+    }
+
+    @Override
+    public void handleException(RuntimeException e) {
+        throw e; // leave exception handling to the servlet container
+    }
+
+    @Override
+    public void close() {
+        // nothing to do for Servlets.
+    }
+
+    /**
+     * This inner class adapts a HttpServletRequest to a PrometheusHttpRequest.
+     */
+    public static class Request implements PrometheusHttpRequest {
+
+        private final HttpServletRequest request;
+
+        /**
+         * Constructs a new Request with the given HttpServletRequest.
+         *
+         * @param request the HttpServletRequest to be adapted
+         */
+        public Request(HttpServletRequest request) {
+            this.request = request;
+        }
+
+        @Override
+        public String getQueryString() {
+            return request.getQueryString();
+        }
+
+
+        @Override
+        public Enumeration<String> getHeaders(String name) {
+            return request.getHeaders(name);
+        }
+
+
+        @Override
+        public String getMethod() {
+            return request.getMethod();
+        }
+
+
+        @Override
+        public String getRequestPath() {
+            StringBuilder uri = new StringBuilder();
+            String contextPath = request.getContextPath();
+            if (contextPath.startsWith("/")) {
+                uri.append(contextPath);
+            }
+            String servletPath = request.getServletPath();
+            if (servletPath.startsWith("/")) {
+                uri.append(servletPath);
+            }
+            String pathInfo = request.getPathInfo();
+            if (pathInfo != null) {
+                uri.append(pathInfo);
+            }
+            return uri.toString();
+        }
+    }
+
+    /**
+     * This inner class adapts a HttpServletResponse to a PrometheusHttpResponse.
+     */
+    public static class Response implements PrometheusHttpResponse {
+
+        private final HttpServletResponse response;
+
+        /**
+         * Constructs a new Response with the given HttpServletResponse.
+         *
+         * @param response the HttpServletResponse to be adapted
+         */
+        public Response(HttpServletResponse response) {
+            this.response = response;
+        }
+
+
+        @Override
+        public void setHeader(String name, String value) {
+            response.setHeader(name, value);
+        }
+
+
+        @Override
+        public OutputStream sendHeadersAndGetBody(int statusCode, int contentLength) throws IOException {
+            if (response.getHeader("Content-Length") == null && contentLength > 0) {
+                response.setContentLength(contentLength);
+            }
+            response.setStatus(statusCode);
+            return response.getOutputStream();
+        }
+    }
+}

prometheus-metrics-exporter-servlet-javax/src/main/java/io/prometheus/metrics/exporter/servlet/javax/PrometheusMetricsServlet.java

@@ -0,0 +1,67 @@
+package io.prometheus.metrics.exporter.servlet.javax;
+
+import io.prometheus.metrics.config.PrometheusProperties;
+import io.prometheus.metrics.exporter.common.PrometheusScrapeHandler;
+import io.prometheus.metrics.model.registry.PrometheusRegistry;
+
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+
+/**
+ * This class extends HttpServlet to create a servlet for exporting Prometheus metrics.
+ * It uses a PrometheusScrapeHandler to handle HTTP GET requests and export metrics.
+ * The servlet can be configured with custom PrometheusProperties and a PrometheusRegistry.
+ */
+public class PrometheusMetricsServlet extends HttpServlet {
+
+    private final PrometheusScrapeHandler handler;
+
+    /**
+     * Default constructor. Uses the default PrometheusProperties and PrometheusRegistry.
+     */
+    public PrometheusMetricsServlet() {
+        this(PrometheusProperties.get(), PrometheusRegistry.defaultRegistry);
+    }
+
+    /**
+     * Constructor with a custom PrometheusRegistry. Uses the default PrometheusProperties.
+     *
+     * @param registry the PrometheusRegistry to use
+     */
+    public PrometheusMetricsServlet(PrometheusRegistry registry) {
+        this(PrometheusProperties.get(), registry);
+    }
+
+    /**
+     * Constructor with custom PrometheusProperties. Uses the default PrometheusRegistry.
+     *
+     * @param config the PrometheusProperties to use
+     */
+    public PrometheusMetricsServlet(PrometheusProperties config) {
+        this(config, PrometheusRegistry.defaultRegistry);
+    }
+
+    /**
+     * Constructor with custom PrometheusProperties and PrometheusRegistry.
+     *
+     * @param config   the PrometheusProperties to use
+     * @param registry the PrometheusRegistry to use
+     */
+    public PrometheusMetricsServlet(PrometheusProperties config, PrometheusRegistry registry) {
+        this.handler = new PrometheusScrapeHandler(config, registry);
+    }
+
+    /**
+     * Handles HTTP GET requests. Exports Prometheus metrics by delegating to the PrometheusScrapeHandler.
+     *
+     * @param request  the HttpServletRequest
+     * @param response the HttpServletResponse
+     * @throws IOException if an I/O error occurs
+     */
+    @Override
+    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
+        handler.handleRequest(new HttpExchangeAdapter(request, response));
+    }
+}

prometheus-metrics-exporter-servlet-javax/version-rules.xml

@@ -0,0 +1,6 @@
+<ruleset comparisonMethod="maven"
+         xmlns="http://mojo.codehaus.org/versions-maven-plugin/rule/2.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://mojo.codehaus.org/versions-maven-plugin/rule/2.0.0 https://www.mojohaus.org/versions-maven-plugin/xsd/rule-2.0.0.xsd">
+    <rules>
+    </rules>
+</ruleset>