JVM in the uWSGI server (updated to 1.9)


As of uWSGI 1.9, you can have a full, thread-safe and versatile JVM embedded in the core. All of the plugins can call JVM functions (written in Java, JRuby, Jython, Clojure, whatever new fancy language the JVM can run) via the RPC subsystem or using uWSGI The uWSGI Signal Framework The JVM plugin itself can implement request handlers to host JVM-based web applications. Currently The JWSGI interface and The Clojure/Ring JVM request handler (Clojure) apps are supported. A long-term goal is supporting servlets, but it will require heavy sponsorship and funding (feel free to ask for more information about the project at info@unbit.it).

Building the JVM support

First of all, be sure to have a full JDK distribution installed. The uWSGI build system will try to detect common JDK setups (Debian, Ubuntu, Centos, OSX…), but if it is not able to find a JDK installation it will need some information from the user (see below). To build the JVM plugin simply run:

python uwsgiconfig.py --plugin plugins/jvm default

Change ‘default’, if needed, to your alternative build profile. For example if you have a Perl/PSGI monolithic build just run

python uwsgiconfig.py --plugin plugins/jvm psgi

or for a fully-modular build

python uwsgiconfig.py --plugin plugins/jvm core

If all goes well the jvm_plugin will be built. If the build system cannot find a JDK installation you will ned to specify the path of the headers directory (the directory containing the jni.h file) and the lib directory (the directory containing libjvm.so). As an example, if jni.h is in /opt/java/includes and libjvm.so is in /opt/java/lib/jvm/i386, run the build system in that way:

UWSGICONFIG_JVM_INCPATH=/opt/java/includes UWSGICONFIG_JVM_LIBPATH=/opt/java/lib/jvm/i386 python uwsgiconfig --plugin plugins/jvm

After a successful build, you will get the path of the uwsgi.jar file. That jarball contains classes to access the uWSGI API, and you should copy it into your CLASSPATH or at the very least manually load it from uWSGI’s configuration.

Exposing functions via the RPC subsystem

In this example we will export a “hello” Java function (returning a string) and we will call it from a Python WSGI application. This is our base configuration (we assume a modular build).

plugins = python,jvm
http = :9090
wsgi-file = myapp.py
jvm-classpath = /opt/uwsgi/lib/uwsgi.jar

The jvm-classpath is an option exported by the JVM plugin that allows you to add directories or jarfiles to your classpath. You can specify as many jvm-classpath options you need. Here we are manually adding uwsgi.jar as we did not copy it into our CLASSPATH. This is our WSGI example script.

import uwsgi

def application(environ, start_response):
    start_response('200 OK', [('Content-Type','text/html')])
    yield "<h1>"
    yield uwsgi.call('hello')
    yield "</h1>"

Here we use uwsgi.call() instead of uwsgi.rpc() as a shortcut (little performance gain in options parsing). We now create our Foobar.java class. Its static void main() function will be run by uWSGI on startup.

public class Foobar {
   static void main() {

       // create an anonymous function
       uwsgi.RpcFunction rpc_func = new uwsgi.RpcFunction() {
           public String function(String... args) {
               return "Hello World";

       // register it in the uWSGI RPC subsystem
       uwsgi.register_rpc("hello", rpc_func);

The uwsgi.RpcFunction interface allows you to easily write uWSGI-compliant RPC functions. Now compile the Foobar.java file:

javac Foobar.java

(eventually fix the classpath or pass the uwsgi.jar path with the -cp option) You now have a Foobar.class that can be loaded by uWSGI. Let’s complete the configuration…

plugins = python,jvm
http = :9090
wsgi-file = myapp.py
jvm-classpath = /opt/uwsgi/lib/uwsgi.jar
jvm-main-class = Foobar

The last option (jvm-main-class) will load a java class and will execute its main() method. We can now visit localhost:9090 and we should see the Hello World message.

Registering signal handlers

In the same way as the RPC subsystem you can register signal handlers. You will be able to call Java functions on time events, file modifications, cron… Our Sigbar.java:

public class Sigbar {
   static void main() {

       // create an anonymous function
       uwsgi.SignalHandler sh = new uwsgi.SignalHandler() {
           public void function(int signum) {
               System.out.println("Hi, i am the signal " + signum);

       // register it in the uWSGI signal subsystem
       uwsgi.register_signal(17, "", sh);

uwsgi.SignalHandler is the interface for signal handlers.

Whenever signal 17 is rased, the corresponding JVM function will be run. Remember to compile the file, load it in uWSGI and to enable to master process (without it the signal subsystem will not work).

The fork() problem and multithreading

The JVM is not fork() friendly. If you load a virtual machine in the master and then you fork() (like generally you do in other languages) the children JVM will be broken (this is mainly because threads required by the JVM are not inherited). For that reason a JVM for each worker, mule and spooler is spawned. Fortunately enough, differently from the vast majority of other platforms, the JVM has truly powerful multithreading support. uWSGI supports it, so if you want to run one of the request handlers (JWSGI, Clojure/Ring) just remember to spawn a number of threads with the --threads option.

How does it work?

uWSGI embeds the JVM using the JNI interface. Unfortunately we cannot rely on JVM’s automatic garbage collector, so we have to manually unreference all of the allocated objects. This is not a problem from a performance and usage point of view, but makes the development of plugins a bit more difficult compared to other JNI-based products. Fortunately the current API simplifies that task.

Passing options to the JVM

You can pass specific options to the JVM using the --jvm-opt option.

For example to limit heap usage to 10 megabytes:

jvm-opt = -Xmx10m

Loading classes (without main method)

We have already seen how to load classes and run their main() method on startup. Often you will want to load classes only to add them to the JVM (allowing access to external modules needing them) To load a class you can use --jvm-class.

jvm-class = Foobar
jvm-class = org/unbit/Unbit

Remember class names must use the ‘/’ format instead of dots! This rule applies to --jvm-main-class too.

Request handlers

Although the Java(TM) world has its J2EE environment for deploying web applications, you may want to follow a different approach. The uWSGI project implements lot of features that are not part of J2EE (and does not implement lot of features that are a strong part of J2EE), so you may find its approach more suited for your setup (or taste, or skills).

The JVM plugin exports an API to allow hooking web requests. This approach differs a bit from “classic” way uWSGI works. The JVM plugin registers itself as a handler for modifier1==8, but will look at the modifier2 value to know which of its request handlers has to manage it. For example the The Clojure/Ring JVM request handler plugin registers itself in the JVM plugin as the modifier2 number ‘1’. So to pass requests to it you need something like that:

http = :9090
http-modifier1 = 8
http-modifier2 = 1

or with nginx:

location / {
    include uwsgi_params;
    uwsgi_modifier1 8;
    uwsgi_modifier2 1;
    uwsgi_pass /tmp/uwsgi.socket;

Currently there are 2 JVM request handlers available:

As already said, the idea of developing a servlet request handler is there, but it will require a sponsorship (aka. money) as it’ll be a really big effort.


  • You do not need special jar files to use UNIX sockets – the JVM plugin has access to all of the uWSGI features.

  • You may be addicted to the log4j module. There is nothing wrong with it, but do take a look at uWSGI’s logging capabilities (less resources needed, less configuration, and more NoEnterprise)

  • The uWSGI API access is still incomplete (will be updated after 1.9)

  • The JVM does not play well in environments with limited address space. Avoid using --limit-as if you load the JVM in your instances.