Setting JDBC statement map cleaner thread ccl to null (#799)

Author: eyalkoren

CHANGELOG.md

@@ -13,6 +13,8 @@
    This makes the runtime attachment work in more environments such as minimal Docker containers.
    Note that the runtime attachment currently does not work for OSGi containers like those used in many application servers such as JBoss and WildFly.
    See the [documentation](https://www.elastic.co/guide/en/apm/agent/java/master/setup-attach-cli.html) for more information.
+ * JDBC statement map is leaking in Tomcat if the application that first used it is udeployed/redeployed. See [this 
+   related discussion](https://discuss.elastic.co/t/elastic-apm-agent-jdbchelper-seems-to-use-a-lot-of-memory/195295).
 
 # Breaking Changes
  * The `apm-agent-attach.jar` is not executable anymore.

apm-agent-plugins/apm-jdbc-plugin/src/main/java/co/elastic/apm/agent/jdbc/helper/JdbcHelper.java

@@ -35,7 +35,17 @@
 public abstract class JdbcHelper {
     @SuppressWarnings("WeakerAccess")
     @VisibleForAdvice
-    public static final WeakConcurrentMap<Object, String> statementSqlMap = new WeakConcurrentMap<>(true);
+    public static final WeakConcurrentMap<Object, String> statementSqlMap;
+
+    static {
+        statementSqlMap = new WeakConcurrentMap<>(true);
+        // This class will probably be loaded in the context of a request processing thread
+        // whose context class loader is the web application ClassLoader.
+        // This leaks the web application ClassLoader when the application is undeployed/redeployed.
+        // Tomcat will then stop the thread because it thinks it was created by the web application.
+        // That means that the map will never be cleared, creating a severe memory leak.
+        statementSqlMap.getCleanerThread().setContextClassLoader(null);
+    }
 
     /**
      * Maps the provided sql to the provided Statement object