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# VS Code support for Java using the javac API
Provides Java support using the javac API.
## Features
### Lint
<img src="http://g.recordit.co/DGTYfpFdSD.gif">
### Autocomplete
<img src="http://g.recordit.co/bCbYuegVRV.gif">
### Go-to-definition
<img src="http://g.recordit.co/Fg7cpH1rnz.gif">
## javaconfig.json
The presence of a `javaconfig.json` file indicates that this directory is the root of a Java source tree.
### Examples
### Maven
You can configure maven to output the current classpath to a file,
classpath.txt, where Visual Studio Code will find it.
#### javaconfig.json
Set the source path, and get the class path from a file:
{
"sourcePath": ["src/main/java"],
"classPathFile": "classpath.txt",
"outputDirectory": "target"
}
#### pom.xml
Configure maven to output `classpath.txt`
<project ...>
...
<build>
...
<plugins>
...
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<version>2.9</version>
<executions>
<execution>
<id>build-classpath</id>
<phase>generate-sources</phase>
<goals>
<goal>build-classpath</goal>
</goals>
</execution>
</executions>
<configuration>
<outputFile>classpath.txt</outputFile>
</configuration>
</plugin>
</plugins>
</build>
</project>
#### .gitignore
Ignore `classpath.txt`, since it will be different on every host
classpath.txt
...
## Structure
### Java service process
A java process that does the hard work of parsing and analyzing .java source files.
pom.xml (maven project file)
src/ (java sources)
repo/ (tools.jar packaged in a local maven repo)
target/ (compiled java .class files, .jar archives)
target/fat-jar.jar (single jar that needs to be distributed with extension)
### Typescript Visual Studio Code extension
"Glue code" that connects to the external java process using a network port,
and implements various features of the Visual Studio Code extension API.
package.json (node package file)
tsconfig.json (typescript compilation configuration file)
tsd.json (project file for tsd, a type definitions manager)
lib/ (typescript sources)
out/ (compiled javascript)
## Design
This extension consists of an external java process,
and some typescript glue code that talks to the VS Code API.
The typescript glue code communicates with the external java process using a randomly assigned network port.
In the future, the javac service process could be used separately by other tools that need Java code insight.
### Java service process
The java service process uses the implementation of the Java compiler in tools.jar,
which is a part of the JDK.
When VS Code needs to lint a file, perform autocomplete,
or some other task that requires Java code insight,
the java service process invokes the Java compiler programatically,
then intercepts the data structures the Java compiler uses to represent source trees and types.
### Incremental updates
The Java compiler isn't designed for incremental parsing and analysis.
However, it is *extremely* fast, so recompiling a single file gives good performance,
as long as we don't also recompile all of its dependencies.
We accomplish this by maintaining a single copy of the Java compiler in memory at all times.
When we want to recompile a file,
we clear that *one* file from the internal caches of the Java compiler,
and then rerun the compiler.
### Multiple javaconfig.json
If you have multiple javaconfig.json files in different subdirectories of your project,
the parent directory of each javaconfig.json will be treated as a separate java root.
A separate java service process will be started for each javaconfig.json.
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