Post date: Apr 24, 2017 9:42:16 PM
The java and javac commands are rarely used by Java programmers... build tools like Maven and Gradle make that mostly unnecessary.
However, as of writing (April 2017), both Maven and Gradle still don't provide full support for Java 9, so if you want to start using Java 9 now, or if you just want to know how it will all work behind the scenes when you finally start using it with your favourite tool, it is a good thing to learn how to call java, javac and jar to manage your Java code.
The purpose of this guide is to show how these command have changed from the previous Java versions (and they did change substantially in Java 9) and how you can already use them to manage your applications. It will also discuss the new tools, jdeps and jlink.
It is assumed that you have at least a little bit of familiarity with previous versions of the java/javac/jar command and with the Java 9 module system.
If you need to learn the basics of the new module system, check The State of the Module System, which gives a pretty good overview of it.
Before getting started with the Java 9 commands, you need to get Java 9!
You can download it from the Oracle website, but I'd recommend using SdkMAN! instead because besides making it trivial to upgrade Java (and many other packages), it lets you switch between Java versions with ease (so you can continue using your current tools without trouble).
You can install SdkMAN! with this command (inspect the bash script here):
curl -s "https://get.sdkman.io" | bash
Then, install Java 9:
Check what's the latest available version with sdk list java
sdk install java 9ea163
Now, as long as you have other Java versions installed, you can switch between them with sdk use java <version>.
With Java 9 installed, the first thing to do is to write some code to try our tools!
If we don't use a module descriptor, everything looks pretty much the same as before!
Take this simple Java class:
src/app/Main.java
package app;
public class Main {
public static void main( String[] args ) {
System.out.println( "Hello Java 9" );
}
}
Now, as we're not really using any Java 9 feature yet, we can pretty much compile it as we always did:
javac -d out src/app/Main.java
This will create a class file at out/app/Main.class. To run that, you do it, again, just like in all previous versions:
java -cp out app.Main
Which prints the expected message, Hello Java 9.
By the way, the "hello world" program takes around 0.1 seconds to run with Java 9 on my machine (it's a few years old Macbook Pro).
Not bad for a language that needs to boot up a whole VM to run.
Now, let's create a Greeting library, also without any Java 9 feature yet, to see how that works.
Create the following file:
greeting/src/lib/Greeting.java
package lib;
public class Greeting {
public String hello() {
return "Hi there!";
}
}
Change the app.Main class to use this mini library:
src/app/Main.java
package app;
import lib.Greeting;
public class Main {
public static void main( String[] args ) {
System.out.println( new Greeting().hello() );
}
}
Compile the Greeting lib:
javac -d greeting/out greeting/src/lib/Greeting.java
We want to show how to use legacy, non-modular, pre-Java 9 libraries, so let's jar this library without a Java 9 module descriptor or anything (you could do this with the Java 8 compiler, or even some earlier version), just to make sure that this still works:
mkdir libs jar cf libs/lib.jar -C greeting/out .
This will create the libs/lib.jar file containing just the lib.Greeting class (and an auto-generated manifest file).
You can inspect the jar with the tf option:
jar tf libs/lib.jar
Which should print:
META-INF/
META-INF/MANIFEST.MF
lib/
lib/Greeting.class
Now to compile app.Main, we need to tell the compiler where to find the lib.Greeting class.
We can do that using the good old cp (classpath) option for now:
javac -d out -cp libs/lib.jar src/app/Main.java
Same thing to run the program:
java -cp out:libs/lib.jar app.Main
In MS Windows, remember to replace the : separator in the command above with ;
We could package the app into a jar as well, of course:
jar cf libs/app.jar -C out .
And then, run it with:
java -cp 'libs/*' app.Main
This is a good time to see what our current project directory structure looks like so far:
.
├── greeting
│ ├── out
│ │ └── lib
│ │ └── Greeting.class
│ └── src
│ └── lib
│ └── Greeting.java
├── libs
│ ├── app.jar
│ └── lib.jar
├── out
│ └── app
│ └── Main.class
└── src
└── app
└── Main.java
So far, nothing new (which is a great thing, as most of what you know is still valid).
But now, let's finally start modularizing our application by creating a module descriptor (always called module-info.java and placed under the root source directory):
src/module-info.java
module com.app {
}
The command to compile a Java 9 module is a little bit different from what we saw previously. Trying to use the same command as before and just adding the module file to the list of files to compile causes an error:
javac -d out -cp 'libs/lib.jar' src/module-info.java src/app/Main.java # doesn't work
src/app/Main.java:3: error: package lib does not exist
import lib.Greeting;
^
src/app/Main.java:7: error: cannot find symbol
System.out.println( new Greeting().hello() );
^
symbol: class Greeting
location: class Main
2 errors
So, what's going on? By just adding the module descriptor, our code no longer compiles. To understand why, we need to understand unnamed modules.
The unnamed module
Any class that is not loaded from a named module is automatically made part of the unnamed module. In our case above, before creating a module descriptor, our code was not part of any module, hence it was associated with the unnamed module. The unnamed module is a compatibility mechanism, basically, that allows code that has not been migrated to Java 9 and modularized yet to be used by Java 9 applications. For this reason, code belonging to the unnamed module have rules akin to Java 8 and earlier: it can see all packages exported by all other modules, and all packages of the unnamed module are exported (ie. visible from other modules).
Once a module descriptor is added to a module, its code is no longer part of the unnamed module and therefore cannot see code from other modules unless it imports them! In the case above, the com.app module does not explicitly require any module, so the greeting library module is not visible to it. It can only "see" the java.base module's packages (which are a mandatory, implicit requirement of all modules).
Java 9 modules, except for the elusive unnamed module discussed above, must declare which other modules they require. In the case of the com.app module, the only requirement is the greeting library. But, as you might have guessed, the greeting library (as well as any of the libraries you use that do not support Java 9 yet) is not a Java 9 module, so how can we declare a requirement on it?
Well, in cases like this, you need to know the name of the library's jar file. That's right, if you have a dependency on a library that has not been converted to use Java 9 modules yet, you need to know what the jar file for that library is called, because Java 9 will convert the file name to a valid Java 9 module name.
This is called an automatic module.
Similarly to unnamed modules, automatic modules can read from all other modules, and all of its packages are exported. But unlike the unnamed modules, it is possible to refer to automatic modules by name from explicit modules.
To figure out the automatic module's name, the compiler converts non-alphabetic characters .'s, so something like the slf4j-api-1.7.25.jar file would become the module name sl4j.api.
We gave the greetings' library jar a pretty bad name: lib.jar. To avoid requiring the lib library, let's rename the jar file to greetings-1.0.jar:
mv libs/lib.jar libs/greetings-1.0.jar
That's a much more standard file name, and now we can tell Java to turn this into an automatic module with a somewhat acceptable name: greetings. And we can require it from the com.app module:
src/module-info.java
module com.app {
requires greetings;
}
Modules are not added to the classpath, like regular jar files... they use the new --module-path (or -p) option. So, we can now compile our modules with the following command:
javac -d out -p 'libs/greetings-1.0.jar' src/module-info.java src/app/Main.java
Now, to run the app.Main class with the java command, we can use the new --module (-m) option, which takes either a module name (in which case the Manifest file of the module must declare the Main-Class attribute) or the pattern module-name/main-class:
java -p 'libs/greetings-1.0.jar:out' -m com.app/app.Main
And, again, this prints the greeting, Hi there!.
To create and use the app.jar as a runnable jar, instead of the out dir, and using shorter options for the java command to run that jar:
jar --create -f libs/app.jar -e app.Main -C out . java -p libs -m com.app
Pretty good! The next step is to modularize the library(ies) used by our application as well.
To modularize a library, you can't do better than to start out by using jdeps, a static analysis tool that is part of the Java SE distribution.
For example, to see the dependencies of our tiny Greetings library:
jdeps -s libs/greetings-1.0.jar
greetings-1.0.jar -> java.base
As expected, the greetings library only depends on the java.base module.
We know that com.app depends on greetings. Let's try to get jdeps to tell us that by removing the module-info.class file from the app.jar file first, then running jdeps on it:
zip -d libs/app.jar module-info.class
deleting: module-info.class
jdeps -s -cp libs/greetings-1.0.jar libs/app.jar
app.jar -> libs/greetings-1.0.jar
app.jar -> java.base
Perfect! But it gets better... we can even ask jdeps to auto-generate a module descriptor for a set of jars. Just tell it where to save the generated files, for example, in generated-mods, and where the jars are:
jdeps --generate-module-info generated-mods libs/greetings-1.0.jar libs/app.jar
This creates two files, generated-mods/app/module-info.java and generated-mods/greetings/module-info.java, with the following contents:
generated-mods/app/module-info.java
module app {
requires greetings;
exports app;
}
generated-mods/greetings/module-info.java
module greetings {
exports lib;
}
Very cool! We can now just add the auto-generated module descriptor for the greetings library in the library's source code, re-package it and have a fully modular hello world application.
javac -d greeting/out $(find greeting/src -name *.java) jar cf libs/greetings-1.0.jar -C greeting/out .
Now we have both the application and the library we were using fully modularized! After removing the generated and binary files, our application structure now looks like this:
├── greeting
│ └── src
│ ├── lib
│ │ └── Greeting.java
│ └── module-info.java
├── libs
│ ├── app.jar
│ └── greetings-1.0.jar
└── src
├── app
│ └── Main.java
└── module-info.java
One final improvement you may want to make is to rename the src folders with the module's full names (or add a new level under src with the module's full names), as that's the new recommendation for Java 9.
Notice that to be able to get good data from jdeps, you must provide the locations of the jars of all transitive dependencies that are used in the application, so that it can create a full module graph.
An easy way to get the list of jars a library requires is to use this simple Gradle script shown below. It will print the location of the local jars for all dependencies of the libraries you add in the dependencies section, downloading them if necessary (this example will show the jars for the JavaSlang library, recently renamed VAVR):
build.gradle
apply plugin: 'java'
repositories {
mavenLocal()
mavenCentral()
}
dependencies {
compile 'io.javaslang:javaslang:2.0.6'
}
task listDeps {
println configurations.compile*.absolutePath.join(' ')
}
If you don't have Gradle, you can use SDKMAN! (or your favourite package manager) to install it:
sdk install gradle
To get the list of dependencies, run:
gradle listDeps
Then, give the output to jdeps for analysis and auto-generation of module metadata.
This is the file jdeps outputs for javaslang.match:
module javaslang.match {
requires transitive java.compiler;
exports javaslang.match;
exports javaslang.match.annotation;
exports javaslang.match.generator;
exports javaslang.match.model;
provides javax.annotation.processing.Processor with
javaslang.match.PatternsProcessor;
}
As easy as it gets, so hopefully most Java libraries will be fully modularized within a short amount of time.
With jlink, Java applications can be distributed as native runtime images that do not require the JVM to be installed in the target system to run.
The following command creates an image for our com.app module without optimisations/compression or stripping debug information:
jlink -p $JAVA_HOME/jmods:libs --add-modules com.app \ --launcher start-app=com.app \ --output dist
A smaller distribution can be obtained by using some of jlink options like --strip-debug and --compress:
jlink -p $JAVA_HOME/jmods:libs --add-modules com.app \ --launcher start-app=com.app \ --strip-debug --compress=2 \ --output small-dist
On my machine, the size of the distributions, according to du -sh, are:
du -sh small-dist dist
21M small-dist
35M dist
To launch the application, use the provided launcher in the bin directory:
dist/bin/start-app
This time, without the help of the local java command, you should see the Hi there! message printed out again from our com.app module, with the help of the greeting library!
And with that, we come to the end of this Java 9 guide. There is a lot more about Java 9 and its module system that has not been covered in this guide, but I hope that this guide will show you most of what you need to know to quickly get started with Java 9. Have fun!