Features

JavaAgent which can be used to intercept invocations selected method (even JDK method)
The agent uses JavaAssist library to modify the code when classes are being loaded.
For the selected methods - every invocation is logged (when method returns)
- Logging is done to standard error
- If method returns normally
  - Method name, all input arguments and return value are logged
- If method throws an exception (Runtime/Checked)
  - Method name all, input arguments and exception are logged
  - The exception is then rethrown (i.e. the behaviour of the method is not modified)
You can use this to troubleshoot complex real-world applications, or just as an example to how to just JavaAssist when writing your own agents.

Why would I need this

It is often difficult or impossible get enough debug information for a specific issue especially when using third-party libraries (or JDK libraries).
It may not be convenient (or even possible) to add extra logging into the code just to understand what's going on.
Example:
- You need to understand when your application is connecting (to which IP)
- What DNS lookups it is making and what IP is is resolving to.
Real life situations:
- Consider your app is connecting to another service which is load-balanced using DNS round-robin and is failing intermittently.
  - In this case you need to know (at least while debugging) what's IP address you are connecting to.
- Consider you want to troubleshoot Kerberos Authentication
  - In this case you can enable Kerberos debugging but unfortunately it does not log the IP it is connecting to.
  - You need to understand the underlying DNS lookups and outbound UDP connections
  - Since the code is in JDK you cannot change it or add your custom logging (at least not easily)

Example Usage

The java code:

    InetAddress.getByName("www.google.com")
    InetAddress.getByName("www.google.con")

Would produce the output (to standard error):

    2017-10-14 17:38:41.379 [main] DebugAgent InetAddress::getByName(www.google.com) => www.google.com/172.217.16.100
    2017-10-14 17:38:41.430 [main] DebugAgent InetAddress::getByName(www.google.con) => java.net.UnknownHostException: www.google.con: Name or service not known

How to run the agent

With default arguments:

java -javaagent:target/debug-agent-0.1-SNAPSHOT.jar \
    -jar application.jar

With all arguments (enabling debug / defining methods to be intercepted explicitly):

java -javaagent:target/debug-agent-0.1-SNAPSHOT.jar=debug=true;methods=java.net.InetAddress::getByName(java.lang.String)|java.net.InetAddress::getByName(java.lang.String, java.net.InetAddress) \
    -jar application.jar

Agent arguments (format: key1=value1;key2=value2)

debug
- indicates whether debug logging should be enabled (disabled by default)
- debug=true means debug should be enabled.
methods
- definition of the methods to be intercepted
- Example:
```
java.net.InetAddress::getByName(java.lang.String)
```
- You can include multiple methods separated by |
- If omitted - if omitted the a DNS lookups, opening TCP/UDP connections is logged.

Contributions

If you wish to contribute and/or submit a pull request please do so on GitLab
The repository is maintained on GitLab https://gitlab.com/janecekt/java-debug-agent
It is mirrored to GitHub as a read-only copy but it is not maintained there any more

License

janecekt/java-debug-agent

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