Language Server for Java using the Java compiler API

A Java language server implemented using the Java compiler API. Requires that you have Java 10 installed on your system.

Installation

Install from the VS Code marketplace

Issues

Features

Javadoc

Signature help

Autocomplete symbols (with auto-import)

Autocomplete members

Go-to-definition

Find symbols

Lint

Type information on hover

Find references

Usage

The language server will provide autocomplete and other features using:

.java files anywhere in your workspace
Java platform classes
External dependencies specified using pom.xml, Bazel, or settings

Settings

If the language server doesn't detect your external dependencies automatically, you can specify them using .vscode/settings.json

{
    "java.externalDependencies": [
        "junit:junit:jar:4.12:test", // Maven format
        "junit:junit:4.12" // Gradle-style format is also allowed
    ]
}

If all else fails, you can specify the java class path manually:

{
    "java.classPath": [
        "lib/some-dependency.jar"
    ]
}

You can generate a list of external dependencies using your build tool:

Maven: mvn dependency:list
Gradle: gradle dependencies

The Java language server will look for the dependencies you specify in java.externalDependencies in your Maven and Gradle caches ~/.m2 and ~/.gradle. You should use your build tool to download the library and source jars of all your dependencies so that the Java language server can find them:

Maven
- mvn dependency:resolve for compilation and autocomplete
- mvn dependency:resolve -Dclassifier=sources for inline Javadoc help

Gradle

gradle dependencies for compilation and autocomplete

Include classifier: sources in your build.gradle for inline Javadoc help, for example:

dependencies {
    testCompile group: 'junit', name: 'junit', version: '4.+'
    testCompile group: 'junit', name: 'junit', version: '4.+', classifier: 'sources'
}

Design

The Java language server uses the Java compiler API to implement language features like linting, autocomplete, and smart navigation, and the language server protocol to communicate with text editors like VSCode.

Incremental updates

The Java compiler API provides incremental compilation at the level of files: you can create a long-lived instance of the Java compiler, and as the user edits, you only need to recompile files that have changed. The Java language server optimizes this further by focusing compilation on the region of interest by erasing irrelevant code. For example, suppose we want to provide autocomplete after print in the below code:

class Printer {
    void printFoo() {
        System.out.println("foo");
    }
    void printBar() {
        System.out.println("bar");
    }
    void main() {
        print // Autocomplete here
    }
}

None of the code inside printFoo() and printBar() is relevant to autocompleting print. Before servicing the autocomplete request, the Java language server erases the contents of these methods:

class Printer {
    void printFoo() {
        
    }
    void printBar() {
        
    }
    void main() {
        print // Autocomplete here
    }
}

For most requests, the vast majority of code can be erased, dramatically speeding up compilation.

Logs

The java service process will output a log file to stderr, which is visible in VSCode using View / Output, under "Java".

Contributing

If you have npm and maven installed, you should be able to install locally using

npm install -g vsce
npm install
./scripts/build.sh

xndcn / java-language-server