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jackplay

★ 85 Open GitHub ↗

A JVM tracing tool that helps you in troubleshooting application behavior.

// repository documentation

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# What is Jackplay?
  Jackplay is a **JVM/Java tracing tool** that helps you troubleshoot application behaviour. It works at JVM level and supports languages like Java, Groovy, Clojure, etc.

It allows you to **trace** method execution in a JVM such that you know exactly the ins and outs coming through your method. It also allows you to **redefine** a method in a JVM **live**! All these comes without any need to change your application code.

If you still wonder what it is for, imagine that you are told that your program does not work in TEST which is a deployed environment. What would you do to troubleshoot? You might be thinking to find out what does a downstream service returns to your service/application, what does your database returns to a query, etc. you then want to add some logging statements and redeploy your program with your code changes such that you can see those results from logs. There you go, **Jackplay gives you *out-of-the-box* and *on-demand* logging functionality without code change or redeployment**.

# NOTE
 - This codebase is not in active maintenance, but I have made the master branch work under JDK 11 as well as JDK 8 (at least you can clone the source, build it yourself). If you find it not working or you want to make it work under other major JDK versions, and you run into issues, please let me know, happy to assist. I can be reached at alfred.xiao@outlook.com.

# Latest Version
  ```com.github.alfredxiao/jackplay-bootstrap 0.10.0```

```com.github.alfredxiao/jackplay-agent 0.10.0```

Can be found via [Maven Repository] (http://search.maven.org/#search%7Cga%7C1%7Cjackplay)
# Features
 - Trace or redefine a method in a JVM live even after a class has been loaded
 - Trace method execution no matter there is exception thrown or not
 - Can turn on or off a method being traced or redefined
 - Can blacklist or whitelist specified packages
 - **No need** to modify application code
 - **No need** to restart your service/application
 - Runs an embedded lightweight HTTP server to serve the control panel in web UI

# Q and A

## Why do I need Jackplay?
1. Developers are not sure where to put log statements and what to log - before running into a problem in an deployed environment.
2. Sometimes we want to apply a quite trivial change to the code to verify something in a quick manner.
3. We are lazy people and do not want to go through the dev/test/deploy cycle again and again, especially in a testing setting.
4. Sometimes we need to trace into a library but it is a challenge since you cannot change its source code.

If you have ever run into any situation as mentioned above, you would benefit from Jackplay's code change free tracing and redefinition capabilities.

## What's the difference between 'Tracing' and 'Debugging'?

1. A debugger is heavyweight, it requires full source code, an IDE and deep knowledge of how the code works. Whereas a tracing tool is lightweight, it does not require full source code or deep knowledge of the code.
2. A debugger stops or pauses execution, whereas a tracing tool like Jackplay does not change application behaviour. A tracing tool's role is like a silent interceptor.
3. A debugger give more insight into how a function body executes, but a tracing tool touches only the entry and exit points of your functions.
4. A debugger normally works at TCP level, and does not provide a UI; Jackplay gives a web UI which is easy to use for developers, testers, and DevOps.

# Usage

## Start you application with jackplay

Add the follow two arguments to your JVM startup command line:

```
  -Xbootclasspath/a:jackplay-bootstrap-<version>.jar -javaagent:jackplay-agent-<version>.jar
  ```

Example:

```
  java -Xbootclasspath/a:jackplay-bootstrap-<version>.jar -javaagent:jackplay-agent-<version>.jar -cp myapplication.jar myapp.Main
  ```

To add custom options:
  e.g. to set port number for the embedded web server to listen on, and specify blacklist, you append these parameters to -javaagent argument, as follows:

```
  -javaagent:jackplay-agent-<version>.jar=port=8282;blacklist=java.net:java.nio
  ```

Note that multiple options are separated by ";" rather than ",".

Also note that package names in 'blacklist' are separated by ':', you can set whitelist as well, but you can't set both blacklist and whitelist at the same time.

Options supported:

| Option           | Description                                                               | Default | Example      |
|------------------|---------------------------------------------------------------------------|---------|--------------|
|*port*            | port number the web server listens on                                     | 8181    |              |
|*logLevel*        | can be either info, debug, or error, default is info                      | info    | error, debug |
|*traceLogLimit*   | how many entries of trace log Jackplay holds in memory and are displayed in its web UI, old log entries are removed when new entries come when we run out of capacity | 300 | |
|*autoSuggestLimit*| specifies the limit of items auto suggestion displays | 100 | |
|*defaultTrace*    | colon separated full method names that we want Jackplay to trace by default, this is a perfect place for tracing application initialization behaviour like configuration loading, etc. | | ```myapp.Main.loadConfig(String[] args)``` |
|*logFile*         | file path to write Jackplay logs to. Note if file size grows over 100M, it will be truncated | | ```./jackplay.log``` |
|*blacklist*       | colon separated packages to not allow tracing or redefining, java.lang is always blacklisted | | ```java.net:myapp.utils``` |
|*whitelist*       | colon separated packages to allow tracing or redefining, once you provide a whitelist, other packages are prevented from being able to be traced or redefined | | |
|*https*           | whether to use https                                                      | false   |              |
|*keystoreFilepath*| when using https, this is used to set the file path to your keystore file |         |              |
|*keystorePassword*| when using https, this is used to set the password for your keystore file |         |              |
|*traceStringLength*| when dealing with method arguments or return value, truncate String(s) that are too long for transferring or display purpose | 36 |  |
|*traceArrayLength*| when dealing with Arrays, only display a limited number of elements | 3 |  |

**Note**:
  1. When *https* is set to true, but no password or keystore path provided, a built-in demo self-signed keystore would be used instead.
  2. When you define *defaultTrace*, the way you specify JVM argument might need to be quoted, e.g.

```
  java -Xbootclasspath/a:../jackplay/dist/jackplay-bootstrap-latest.jar -javaagent:../jackplay/dist/jackplay-agent-latest.jar="logLevel=debug;blacklist=java.net:java.nio;defaultTrace=myapp.greeter.NiceGreeter.greet(java.lang.String):myapp.Demo.main(java.lang.String[])" -cp . myapp.Demo
  ```

3. *logFile* is used to write debug/info/error log from the Jackplay Java agent, not the trace logs themselves.

## Open Jackplay control panel

Open your [http://yourserver:8181] (http://yourserver:8181)

## To trace a method

Type a class or method name, Jackplay should automatically suggest matching classes/methods. Select one method, then click 'Trace'.

## To redefine a method

Click 'Redefine...', similarly, choose a method, then key in new method body. The method body can only be written in Java, and must be enclosed with {}.

Example:

```
   {
     java.util.List myList = new java.util.ArrayList();
     myList.add("A");
     myList.add("B");

return myList;
   }
   ```

# How to Run the provided Demo app

1. Clone project, and build jackplay

```
  git clone https://github.com/alfredxiao/jackplay.git
  cd jackplay/jackplay
  ant clean dist
  ```

2. Run Demo (assuming you current work directory is jackplay/jackplay)

```
  cd ../demo-java
  ./run.sh
  ```

3. Open browser and point to Jackplay page

Open [http://localhost:8181](http://localhost:8181), and try to play with it.

4. Try out tracing, for Example

In the 'Method:' input box, type 'RudeGreeter', select 'myapp.greeter.RudeGreeter.greet(java.lang.String)' and click 'Trace'.

# Limitations

- Cannot trace or redefine native, abstract method, or a method in an Interface, because they don't have body.
 - Does not support constructor method yet
 - When redefining a method, you can only use Java language. And you better avoid using advanced Java features like Lambda, Generics, etc. For details, see [javassist page](https://jboss-javassist.github.io/javassist/tutorial/tutorial2.html#limit).
 - It is recommended to use *latest* **Chrome** to access the web UI.
 - Supports JDK 8 (not JDK 7 yet)

# License

[Apache License 2.0](http://www.apache.org/licenses/LICENSE-2.0)

# Contact

If you have any questions, suggestion, feel free to reach me at alfred.xiao@outlook.com .

jackplay

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