Table Of Content

1. Overview
2. Features
3. Requirement
4. Usage
5. watcher.config format
6. References

Overview

A simple light weight Java utility to monitor directory for any change and optionally execute post processor. Post processor can be a Java class, python script or shell script. Post processor is program (java/python/shell script) which will be triggered by Watcher when a certain event (create, delete, modify) occurs as configured for the watcher.

Features

• Monitor one or more directories simultaneously through configuration. Configuration can be provided either via command line or configuration properties file.
• Command Line Configuration: You can configure watcher using command line arguments. This is the simplest way of configuring watcher. Command line arguments can be mixed with configuration properties file as well. In such case if any configuration conflicts with both command line and configuration properties file, command line arguments takes precedence. Command line arguments are set as system properties. List of supported system properties are same as the keys which are defined under watcher.config format section.
• Configuration Properties File: You can provide elaborate configuration using a properties file. With this you can even configure multiple directories to be monitored and post processed differently (or not at all). Refer watcher.config format section for complete details.
• Utility can also optionally generate log files capturing each event and affected resource - directory or file. File is a CSV with timestamp,event,resource_name format. If log file is not configured, Watcher will dump this same information on standard output (console).
• Utility also takes care of monitoring any new file created under the directory being monitored or even any new directory created in case of recursive flag is set to true. Refer watcher.config format for details.

Requirement

• Main requirement is you need to have Java 7 or higher runtime to execute Watcher.
• If you are configuring Java post processor then
  • You should provide fully qualified class name and optionally method to invoke. Refer watcher.config format section.
  • If no method is provided, default method i.e. main will be invoked.
  • If you want to invoke custom method with no parameters just provide the name.
  • Its also possible to receive event name and/or affected resource as method parameter. In such case you need to provide method name which takes one/two string parameter(s).
• If you are using Python post processor then
  • Ensure Python is installed and python interpreter is in PATH so watcher can invoke it.
  • You should provide the complete file system path to the script which should be invoked. Refer watcher.config format section for details.
  • Event name and affected resource will be passed as script parameters.
• If you are using Shell Script post processor then
  • Ensure shell/bash script you want to trigger has execute permission.
  • You should provide the complete file system path to the script which should be invoked. Refer watcher.config format section for details.
  • Event name and affected resource will be passed as script parameters.

Usage

• Using maven - mvn exec:java -Dexec.mainClass=com.jak.sandbox.watcher.Main -Dwatcher.config=<PathTo_wrapper_config_file>
• Using standalone jar file as self executable
  • You can build the binary using maven by invoking mvn clean package
  • Alternatively you can download already packaged binary from bin directory of the project.
  • Once you have jar file built, use java -Dwatcher.config=<PathTo_wrapper_config_file> -jar wrapper-<MAJOR>.<MINOR>.jar

watcher.config format

• watch.config is a properties file holding configuration for one or more watchers.
• Watcher is a set of properties defining configuration of directory which you want to monitor. Below configuration defines one watcher.
• watcher.<name>.watch=<dir_path>: Defines which directory to monitor.
  • Optional: false
  • <name>: Name of the watcher.
  • <dir_path>: Path to directory to monitor
  • Example: watcher.java.watch=/tmp/tmp1
• watcher.<name>.events: Defines which all events to monitor
  • Optional: true.
  • Possible events - CREATE,MODIFY,DELETE (case sensitive)
  • You can define one or multiple events. Multiple events are defined as comma separated string.
  • If this key is not defined, all the events will be monitored.
  • Example: watcher.java.events=DELETE,MODIFY,CREATE
• watcher.<name>.recursive: Defines if defined directory to be monitored recursively.
  • Optional: true
  • If true, along with the directory defined as watcher.<name>.watch, all the subdirectories will also be monitored. This includes any new directory which is created after the watcher started monitoring.
  • If false, only the defined directory will be monitored.
  • If this key is not defined, only defined directory will be monitored.
  • Example: watcher.java.recursive=true
• watcher.<name>.logfile=<dir_path>
  • Optional: true
  • Watcher can generate log file capturing each event and affected resource with timestamp of the event.
  • If this key is defined, watcher will generate a log file under the directory defined. Name of the log file will be <name_of_watcher>.log
  • If this key is not defined, watcher will dump this same information on standard console.
  • Example: watcher.java.logfile=/tmp/watcherlog
• watcher.<name>.eventprocessor.type=<type>: Type of the event post processor
  • Optional: true
  • Supported values: java/python/shell
  • Example: watcher.java.eventprocessor.type=java
• Type java:
  • watcher.<name>.eventprocessor.class=<class_name>
    • Optional: conditional - Required if watcher.<name>.eventprocessor.type is defined and value is defined as java.
    • <class_name>: Fully qualified class name. Ensure that this class and any other required dependencies are in classpath of watcher.
    • Example: watcher.java.eventprocessor.class=com.jak.sandbox.sample.Sample
  • watcher.<name>.eventprocessor.method=<method_name>[,EVENT/RESOURCE][,RESOURCE/EVENT]:
    • Optional: true
    • Considered only if watcher.<name>.eventprocessor.type is defined and value is defined as java.
    • If this key is no defined, watcher will invoke default main method of the class.
    • If this key is defined...
      • <method_name>: Name of the method to invoke.
      • If event name and/or resource name are required they can be defined with keywords EVENT & RESOURCE.
    • Examples
      1. watcher.java.eventprocessor.method=someMethod: Invokes someMethod method with no parameters.
      2. watcher.java.eventprocessor.method=someMethod,EVENT: Invokes someMethod method with one String parameter which will hold name of the event - CREATE/DELETE/MODIFY.
      3. watcher.java.eventprocessor.method=someMethod,RESOURCE,EVENT: Invokes someMethod method with two String parameters. First parameter will hold value of the affected resource and second parameter will hold value as the name of the event.
• Type python:
  • watcher.<name>.eventprocessor.script=<script_path>: Defines script name with full path on file system which will be invoked as python post processor.
    • Optional: conditional - Required if watcher.<name>.eventprocessor.type is defined and value is defined as python.
    • Ensure that python is installed and is in PATH so that watcher can invoke python script with python interpreter
    • Example: watcher.python.eventprocessor.script=/tmp/scripts/sample.py
• Type shell:
  • watcher.<name>.eventprocessor.script=<script_path>: Defines script name with full path on file system which will be invoked as shell script post processor.
    • Optional: conditional - Required if watcher.<name>.eventprocessor.type is defined and value is defined as shell.
    • Ensure that the shell/bash script you want to execute has execute permission.
    • Example: watcher.shell.eventprocessor.script=/tmp/scripts/sample.sh
• Time Config: Defines for how long to monitor the resources. This is global setting i.e. applies to all configured watchers.
  • watcher.watch.time.duration: Number defining how many sec/min/hours/days you want watcher to monitor. Optional: true Default: 1
  • watcher.watch.time.unit: Unit of time.
    • Optional: true
    • Default: M
    • Possible values
      • S = seconds
      • M = minutes
      • H = hours
      • D = Days

References

• Watch Service: Good reference on how to use WatcherService introduced with Java 7. This tool uses the code from the example provided in this link.
• If you want to setup the project and compile/build, you will need Java 7 - JDK. It can be downloaded from here
• If you want to only run the binary donwloaded from bin directory of the project then you will only need Java 7 runtime (JRE) which can be dowloaded from here.
• There is a similar lightweight utility - fswatch - written in C++11 by Karel. You can find the details of fswatch over here. In fact this inspired me to comlete Watcher which was just some quick-and-dirty code I hacked sometime back to troubleshoot some problem on one of the projects I was working.