## What do these changes do?
Previously, Java worker configuration is complicated, because it requires setting environment variables as well as command-line arguments.
This PR aims to simplify Java worker's configuration.
1) Configuration management is now migrated to [lightbend config](https://github.com/lightbend/config), thus doesn't require setting environment variables.
2) Many unused config items are removed.
3) Provide a simple `example.conf` file, so users can get started quickly.
4) All possible options and their default values are declared and documented in `ray.default.conf` file.
This PR also simplifies and refines the following code:
1) The process of `Ray.init()`.
2) `RunManager`.
3) `WorkerContext`.
### How to use this configuration?
1. Copy `example.conf` into your classpath and rename it to `ray.conf`.
2. Modify/add your configuration items. The all items are declared in `ray.default.conf`.
3. You can also set the items in java system prosperities.
Note: configuration is read in this priority:
System properties > `ray.conf` > `ray.default.conf`
## Related issue number
N/A
* Enable java worker support
--------------------------
This commit includes a tailored version of the Java worker implementation from Ant Financial.
The changes for build system, python module, src module and arrow are in other commits, this commit consists of the following modules:
- java/api: Ray API definition
- java/common: utilities
- java/hook: binary rewrite of the Java byte-code for remote execution
- java/runtime-common: common implementation of the runtime in worker
- java/runtime-dev: a pure-java mock implementation of the runtime for fast development
- java/runtime-native: a native implementation of the runtime
- java/test: various tests
Contributors for this work:
Guyang Song, Peng Cao, Senlin Zhu,Xiaoying Chu, Yiming Yu, Yujie Liu, Zhenyu Guo
* change the format of java help document from markdown to RST
* update the vesion of Arrow for java worker
* adapt the new version of plasma java client from arrow which use byte[] instead of custom type
* add java worker test to ci
* add the example module for better usage guide
