Refactor Java agent to let applications manage the log handlers

[deliverable/lttng-ust.git] / doc / java-agent.txt

diff --git a/doc/java-agent.txt b/doc/java-agent.txt
index 3fc6c4931e495fa2195fc7480c78af4d35ecea8b..b5f722c7d6a8e06ea1f2a54dde2453545e57934a 100644 (file)
--- a/doc/java-agent.txt
+++ b/doc/java-agent.txt
@@ -1,3 +1,7 @@
+======================
+ Using the Java agent
+======================
+
 The agent can be built in three different configurations:
 
 1) Java agent with JUL support:
@@ -32,18 +36,56 @@ application's classpath.
 Both logging libraries also require an architecture-specific shared object
 (e.g: "liblttng-ust-jul-jni.so"), which is installed by the build system when
 doing "make install". Make sure that your Java application can find this shared
-object with the "java.library.path" property.
+object, by using the "java.library.path" property if necessary.
+
+In order to use UST tracing in your Java application, you simply need to
+instantiate a LttngLogHandler or a LttngLogAppender (for JUL or Log4j,
+respectively), then attach it to a JUL or Log4j Logger class.
+
+Refer to the code examples in examples/java-jul/ and examples/java-log4j/.
+
+LTTng session daemon agents will be initialized as needed. If no session daemon
+is available, the execution will continue and the agents will retry connecting
+every 3 seconds.
+
+
+==============
+ Object model
+==============
+
+The object model of the Java agent implementation is as follows:
+
+---------
+Ownership
+---------
+Log Handlers: LttngLogHandler, LttngLogAppender
+  n handlers/appenders, managed by the application.
+  Can be created programmatically, or via a configuration file,
+  Each one registers to a specific agent singleton (one per logging API) that is loaded on-demand
+
+Agent singletons: LttngJulAgent, LttngLog4jAgent
+  Keep track of all handlers/appenders registered to them.
+  Are disposed when last handler deregisters.
+  Each agent instantiates 2 TCP clients, one for the root session daemon, one for the user one.
+  One type of TCP client class for now. TCP client may become a singleton in the future.
+
+-------
+Control
+-------
+Messages come from the session daemon through the socket connection.
+Agent passes back-reference to itself to the TCP clients.
+Clients use this reference to invoke callbacks, which modify the state of the agent (enabling/disabling events, etc.)
+
+---------
+Data path
+---------
+Log messages are generated by the application and sent to the Logger objects,
+which then send them to the Handlers.
 
-In order to enable the agent in your Java application, you simply have to add
-this as early as you can in the runtime process.
+When a log event is received by a Handler (publish(LogRecord)), the handler
+checks with the agent if it should log it or not, via
+ILttngAgent#isEventEnabled() for example.
 
-import org.lttng.ust.agent.LTTngAgent;
-[...]
-       private static LTTngAgent lttngAgent;
-       [...]
-       lttngAgent = LTTngAgent.getLTTngAgent();
+Events that are logged call the native tracepoint through JNI, which generates
+a UST event. There is one type of tracepoint per domain (Jul or Logj4).
 
-This will initialize automatically the singleton LTTngAgent, and will
-return when the session daemon registration is done. If no session daemon is
-available, the execution will continue and the agent will retry every
-3 seconds.