Implement ToolProvider SPI

[sormuras]

Implement java.util.spi.ToolProvider and deploy it via a META-INF/services entry and via a module descriptor.

The main class for the Java formatter CLI looks like it is already well-prepared to offer a ToolProvider implementation:

google-java-format/core/src/main/java/com/google/googlejavaformat/java/Main.java

Line 52 in 24ec96c

public Main(PrintWriter outWriter, PrintWriter errWriter, InputStream inStream) {

[sormuras]

Happy to provide a PR for this feature.

[cushon]

naive question: what use-cases would this cover that depending on g-j-f as a library and using com.google.googlejavaformat.java.Formatter doesn't handle?

[sormuras]

My use-cases are (build) tools that want to provide access to other tools (provided) re-using their CLI (option parsing).

Having a programmatic entry point, like com.google.googlejavaformat.java.Formatter or jdk.security.jarsigner.JarSigner does not give the same user experience out of the box -- I'd have to rewrite and maintain the option parsing logic inside a custom ToolProvider implementation.

My current work around is to start an external process for non-ToolProvider-implementing tools:

• forax/pro ... FormatterPlugin
• sormuras/bartholdy ... GoogleJavaFormat

Hence java.util.spi.ToolProvider,

An interface for command-line tools to provide a way to be invoked without necessarily starting a new VM.

comes in handy for my (build) tool(s): 🎼 sormuras/bach#201

[cushon]

Sorry for the delay here, I'm open to trying this out. @sormuras any chance you're still interested in providing a PR for this?

[sormuras]

Sure.

[sormuras]

Filed PR #656 for class-path and automatic module usage.

Writing a compiling module descriptor for this project is not that easy. For example, Google Auto, does not play nice and also the --exports directives used for testing (in Surefire's configuration) must already be applied to javac's call. Here's the outline I started (and failed) with:

module com.google.googlejavaformat {
  exports com.google.googlejavaformat;
  exports com.google.googlejavaformat.java;

  requires com.google.common;
  requires com.google.errorprone.annotations;
  requires jdk.compiler;
  requires org.checkerframework.checker.qual;

  provides java.util.spi.ToolProvider with
      com.google.googlejavaformat.java.FormatToolProvider;
}

[sormuras]

Done via #656 (i.e. 2c5d5fd).