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# Language Server for Java using the [Java compiler API](https://docs.oracle.com/javase/10/docs/api/jdk.compiler-summary.html)
A Java [language server](https://github.com/Microsoft/vscode-languageserver-protocol) implemented using the Java compiler API.
[](https://circleci.com/gh/georgewfraser/java-language-server)
## Installation
[Install from the VS Code marketplace](https://marketplace.visualstudio.com/items?itemName=georgewfraser.vscode-javac)
## [Issues](https://github.com/georgewfraser/java-language-server/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)
## Settings
If the language server doesn't detect your external dependencies automatically, you can specify them using [.vscode/settings.json](https://code.visualstudio.com/docs/getstarted/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:
```json
{
"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](https://docs.oracle.com/javase/10/docs/api/jdk.compiler-summary.html) to implement language features like linting, autocomplete, and smart navigation, and the [language server protocol](https://github.com/Microsoft/vscode-languageserver-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:
```java
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:
```java
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
At the time of this writing, the build only works on Mac, because of the way it uses JLink. However, it would be straightforward to fix this by changing `scripts/link_mac.sh` to be more like `scripts/link_windows.sh`.
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