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Connect a Scala REPL to running JVM processes without any prior setup

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This tool allows you to connect a Scala REPL console to running Oracle (Sun) JVM processes without any prior setup at the target process.

View demo video on YouTube


For Scala 2.12, download

For Scala 2.10 and 2.11, download

Extract the ZIP file, you will see:


scala-library-2.12.8.jar scala-compiler-2.12.8.jar scala-reflect-2.12.8.jar

scala-library, scala-compiler, and scala-reflect of the correct version that your JVM process is using will be loaded, if they have not been loaded yet. The REPL console needs these libraries to work.

For example, your process has already loaded scala-library 2.12.8 by itself, but scala-compiler and scala-reflect haven't been loaded, Scalive will automatically load their version 2.12.8.

If none of them has been loaded, i.e. your process doesn't use Scala, Scalive will load the lastest version in the directory.

For your convenience, Scala 2.12.8 JAR files have been included above.

If your process uses a different Scala version, you need to manually download the corresponding JARs from the Internet and save them in the same directory as above.


Run the shell script

(*nix) or

Run without argument to see the list of running JVM process IDs on your local machine:


Example output:

JVM processes:
#pid    Display name
13821   demos.Boot
17978   quickstart.Boot

To connect a Scala REPL console to a process:


Just like in normal Scala REPL console, you can:

  • Use up/down arrows keys to navigate the console history
  • Use tab key for completion

How to load your own JARs to the process

Scalive only automatically loads

, and
to the system classpath.

If you want to load additional classes in other JARs, first run these in the REPL console to load the JAR to the system class loader:

val cl            = ClassLoader.getSystemClassLoader.asInstanceOf[]
val jarSearchDirs = Array("/dir/containing/the/jar")
val jarPrefix     = "mylib"  // Will match "mylib-xxx.jar", convenient when there's version number in the file name
scalive.Classpath.findAndAddJar(cl, jarSearchDirs, jarPrefix)

Now the trick is just quit the REPL console and connect it to the target process again. You will be able to use your classes in the JAR normally:


Note that

doesn't work.

How Scalive works

Scalive uses the Attach API to tell the target process to load an agent.

Inside the target progress, the agent creates a REPL interpreter and a TCP server to let the Scalive process connect and interact with the interpreter. The Scalive process acts as a TCP client. There are 2 TCP connections, one for REPL data and one for tab key completion data.

Similar projects:

Known issues

For simplicity and to avoid memory leak when you attach/detach many times, Scalive only supports processes with only the default system class loader, without additional class loaders. Usually they are standalone JVM processes, like Play or Xitrum in production mode.

Processes with multiple class loaders like Tomcat are currently not supported.

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