Java/C++ PC Standby Detect and Prevent

Tuesday Apr 6th 2004 by Tomer Petel

Shows how to detect whether the computer is going to go into sleep (standby) mode and allow/disallow it.


This article shows how your Java (and C++) program can detect and refuse sleep/standby/hibernate requests made by the user or the system.


I wrote an application that is fed by a special PCI card, and runs 24/7. We found that if the PC went into standby mode, the PCI card was falsely activated, which was really bad. There was no easy way to uniformly disable the sleep/standby/hibernate features in Windows XP. Therefore, I decided to use the Windows API to detect whether a standby request is made and refuse it. Luckily, the Win32 API has a message, called WM_POWERBROADCAST, that is sent to all applications when the PC wants to go into standby or hibernate mode. The win32 subsystem will abort the standby if you return BROADCAST_QUERY_DENY from this message handler.

By using this API, the enclosed code is able to detect whether the computer goes into standby mode and, through the utilization of JNI, notify the rest of the Java code about that event. Also, the code lets you allow or disallow the PC to enter standby mode.

Using the Code

To use the code from Java, simply instantiate a StandByDetector object and provide the constructor with a reference to a StandByRequestListener. The listener will be notified when a standby/hibernate request is made on the PC. Use the setAllowStandby() to tell the code whether or not you want to allow entering a standby. Remember that the DLL (enclosed) has to be in your java.library.path path. Here is an example:

StandByDetector sd=new StandByDetector(new StandByRequestListener() {
   public void standByRequested() {
      System.out.println("standby requested");

To compile and run the demo:

  1. Extract the ZIP file contents into c:\temp, preserving the ZIP dir structure.
  2. Open cmd and cd into temp. Then, issue the following on the command line:
  3. javac com\ha\common\windows\*.java 
  4. Then, to run the demo, issue on the command line:
  5. java -Djava.library.path=C:\temp\com\ha\common\windows -classpath
  6. Now try to put the PC in standby: Task manager->Shut Down->Stand By.
  7. You will note that the cmd shell shows the message "standby requested" and the PC does not go into standby.
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