Add dynamic link for byte-buddy-agent in exception message

relates to #326

stagemonitor-core/src/main/java/org/stagemonitor/core/instrument/AgentAttacher.java

@@ -16,6 +16,7 @@
 import org.stagemonitor.core.Stagemonitor;
 import org.stagemonitor.core.metrics.health.ImmediateResult;
 import org.stagemonitor.core.util.ClassUtils;
+import org.stagemonitor.core.util.VersionUtils;
 import org.stagemonitor.util.IOUtils;
 
 import java.io.File;
@@ -122,13 +123,21 @@ private static boolean initInstrumentation() {
 			final String msg = "Failed to perform runtime attachment of the stagemonitor agent. Make sure that you run your " +
 					"application with a JDK (not a JRE)." +
 					"To make stagemonitor work with a JRE, you have to add the following command line argument to the " +
-					"start of the JVM: -javaagent:/path/to/byte-buddy-agent-<version>.jar";
+					"start of the JVM: -javaagent:/path/to/byte-buddy-agent-<version>.jar. " +
+					"The version of the agent depends on the version of stagemonitor. " +
+					"You can download the appropriate agent for the stagemonitor version you are using here: " + getByteBuddyAgentDownloadUrl();
 			healthCheckRegistry.register("Agent attachment", ImmediateResult.of(HealthCheck.Result.unhealthy(msg)));
 			logger.warn(msg, e);
 			return false;
 		}
 	}
 
+	private static String getByteBuddyAgentDownloadUrl() {
+		final String groupId = "net.bytebuddy";
+		final String byteBuddyVersion = VersionUtils.getVersionFromPomProperties(ByteBuddy.class, groupId, "byte-buddy");
+		return VersionUtils.getMavenCentralDownloadLink(groupId, "byte-buddy-agent", byteBuddyVersion);
+	}
+
 	private static void ensureDispatcherIsAppendedToBootstrapClasspath(Instrumentation instrumentation)
 			throws ClassNotFoundException, IOException {
 		final ClassLoader bootstrapClassloader = ClassLoader.getSystemClassLoader().getParent();

stagemonitor-core/src/main/java/org/stagemonitor/core/util/VersionUtils.java

@@ -0,0 +1,24 @@
+package org.stagemonitor.core.util;
+
+import org.stagemonitor.configuration.source.PropertyFileConfigurationSource;
+
+import java.util.Properties;
+
+public final class VersionUtils {
+
+	private VersionUtils() {
+	}
+
+	public static String getVersionFromPomProperties(Class clazz, String groupId, String artifactId) {
+		final String classpathLocation = "META-INF/maven/" + groupId + "/" + artifactId + "/pom.properties";
+		final Properties pomProperties = PropertyFileConfigurationSource.getFromClasspath(classpathLocation, clazz.getClassLoader());
+		if (pomProperties != null) {
+			return pomProperties.getProperty("version");
+		}
+		return null;
+	}
+
+	public static String getMavenCentralDownloadLink(String groupId, String artifactId, String version) {
+		return String.format("http://central.maven.org/maven2/%s/%s/%s/%s-%s.jar", groupId.replace('.', '/'), artifactId, version, artifactId, version);
+	}
+}

stagemonitor-core/src/test/java/org/stagemonitor/core/util/VersionUtilsTest.java

@@ -0,0 +1,32 @@
+package org.stagemonitor.core.util;
+
+import net.bytebuddy.ByteBuddy;
+
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class VersionUtilsTest {
+
+	private static final String BYTEBUDDY_GROUP = "net.bytebuddy";
+	private static final String VERSION_PATTERN = "\\d\\.\\d+\\.\\d+";
+
+	@Test
+	public void getVersionFromPomProperties() throws Exception {
+		assertThat(VersionUtils.getVersionFromPomProperties(ByteBuddy.class, BYTEBUDDY_GROUP, "byte-buddy")).matches(VERSION_PATTERN);
+	}
+
+	@Test
+	public void testGetMavenCentralDownloadLink() throws Exception {
+		assertThat(VersionUtils.getMavenCentralDownloadLink(BYTEBUDDY_GROUP, "byte-buddy-agent", "1.7.5"))
+				.isEqualTo("http://central.maven.org/maven2/net/bytebuddy/byte-buddy-agent/1.7.5/byte-buddy-agent-1.7.5.jar");
+	}
+
+	@Test
+	public void testByteBuddyDownloadUrl() throws Exception {
+		final String byteBuddyVersion = VersionUtils.getVersionFromPomProperties(ByteBuddy.class, BYTEBUDDY_GROUP, "byte-buddy");
+		final String mavenCentralDownloadLink = VersionUtils.getMavenCentralDownloadLink(BYTEBUDDY_GROUP, "byte-buddy-agent", byteBuddyVersion);
+		assertThat(mavenCentralDownloadLink)
+				.isEqualTo("http://central.maven.org/maven2/net/bytebuddy/byte-buddy-agent/" + byteBuddyVersion + "/byte-buddy-agent-" + byteBuddyVersion + ".jar");
+	}
+}

stagemonitor-web-servlet/src/main/java/org/stagemonitor/web/servlet/filter/HttpRequestMonitorFilter.java

@@ -103,11 +103,12 @@ private void doMonitor(HttpServletRequest request, HttpServletResponse response,
 
 		try {
 			monitorRequest(filterChain, request, responseWrapper);
+		} catch (Exception e) {
+			handleException(e);
+		} finally {
 			if (isInjectContentToHtml(request)) {
 				injectHtml(response, request, httpServletResponseBufferWrapper);
 			}
-		} catch (Exception e) {
-			handleException(e);
 		}
 	}