Refresh printers in Java while application is running

Question

As the title says, I would like to refresh the printers that are registered in the settings of the computer while my Java application is running. Normally, I can use PrinterJob.lookupPrintServices() to get the printers. However, these are only refreshed when restarting the application. I've read something on that lookupPrintServices() should be done in a new thread in order to get the printers. This however did not work, the list of printers remains the same. The following link shows that this problem should have been fixed in Java 5.0, am I doing something wrong?

Any help is much appreciated!

EDIT Added MWE.

public class MTPrinterTest extends Thread {
    public static void main(String[] args) {
        MTPrinterTest t1 = new MTPrinterTest();
        t1.start();

        try {
            System.in.read();
        } catch (Exception e){}

        MTPrinterTest t2 = new MTPrinterTest();
        t2.start();
    }
    public void run() {
        PrinterJob printerJob;
        PrintService[] printServices;

        printerJob = PrinterJob.getPrinterJob();
        printServices = printerJob.lookupPrintServices();
        System.out.println("Number of servies found: " + printServices.length);
        for (int i =0; i< printServices.length; i++)
            System.out.println("--> Available Printer " + i + ": " + printServices[i]);
        printerJob.printDialog(); 
    } 
}

Answer 1

There is no need to restart the application in order to refresh the list of print services.

Here I found the solution:

/**
 * Printer list does not necessarily refresh if you change the list of 
 * printers within the O/S; you can run this to refresh if necessary.
 */
public static void refreshSystemPrinterList() {
    Class<?>[] classes = PrintServiceLookup.class.getDeclaredClasses();
    for (Class<?> clazz : classes) {
        if ("javax.print.PrintServiceLookup$Services".equals(clazz.getName())) {
            // sun.awt.AppContext.getAppContext().remove(clazz);
            // Use reflection to avoid "Access restriction" error message
            try {
                Class<?> acClass = Class.forName("sun.awt.AppContext");
                Object appContext = acClass.getMethod("getAppContext").invoke(null);
                acClass.getMethod("remove", Object.class).invoke(appContext, clazz);
            } catch (Exception e) {
            }
            break;
        }
    }
}

Basically, the static class PrintServiceLookup.Services maintains the list of print services. So, if you remove this class from the AppContext, you force PrintServiceLookup to create a new instance again. Thus, the list of print services gets refreshed.

Answer 2

I have encountered the same problem before and after multiple tests, it seems like the printer list is snapshotted at the start of the Java application and can't be refreshed after that using java's lookupPrintServices().

What I did to solve that problem is call directly the Winspool API using JNA. If you intend to do so, the Winspool API is well documented by Microsoft : Winspool API documentation

Also, I described a part of my solution to a problem I had a few month ago in this question, it might help you understand JNA and the Winspool API.

Answer 3

A quick workaround is for CUPS-based systems is:

System.setProperty("sun.java2d.print.polling", "false");

Warning, this has the side-effect of gradually slowing down calls (e.g. 80ms) to PrintServiceLookup.lookupPrintServices(...).

Specifically:

Property sun.java2d.print.polling set to true:

• <1ms to call PrintServiceLookup.lookupPrintServices(...)

Property sun.java2d.print.polling set to false:

• 80ms to call PrintServiceLookup.lookupPrintServices(...)

Although 80ms is a very short amount of time for most UI, in a high-load scenario this can affect performance.

Furthemore, this 80ms gradually increases over time. For example, 100,000 calls to PrintServiceLookup.lookupPrintServices(...) gradually increases the delay from 80ms to 1,000ms. Long-running programs may experience a noticeable delay.

That said, this delay is still prefered over the AppContext workaround, especially on CUPS systems (Linux, Mac). The AppContext provided in other solutions introduces JVM threading issues that eventually cause 2,000ms delays, and at times up to 200,000ms delays (reference: https://github.com/qzind/tray/issues/479)

Detailed bug report: https://github.com/AdoptOpenJDK/openjdk-build/issues/1212

Answer 4

Based on this solution link proposed by tresf of the problem link and in a previous answer of IvanRF, I created a new source code which is the merge of both codes.

The purpose of this code in addition to solving the problem is to eliminate the warning:

AppContext is internal proprietary API and may be removed in a future release

public static boolean refreshSystemPrinters() {
    try {
        for (final Thread thread : Thread.getAllStackTraces().keySet()) {
            if (thread.getName().equals("PrinterListener")) {
                thread.interrupt();
            }
        }
        final Class<?> appContextClass = Class.forName("sun.awt.AppContext");
        final Object appContext = appContextClass.getMethod("getAppContext").invoke(null);
        appContextClass.getMethod("put", Object.class, Object.class).invoke(appContext, javax.print.PrintServiceLookup.class.getDeclaredClasses()[0], null);
        javax.print.PrintServiceLookup.lookupPrintServices(null, null);
        return true;
    } catch (final Exception e) {
        e.printStackTrace();
    }
    return false;
}

This code using Java reflection:

final Class<?> appContextClass = Class.forName("sun.awt.AppContext");
final Object appContext = appContextClass.getMethod("getAppContext").invoke(null);
appContextClass.getMethod("put", Object.class, Object.class).invoke(appContext, javax.print.PrintServiceLookup.class.getDeclaredClasses()[0], null);

is equivalent than this:

sun.awt.AppContext.getAppContext().put(javax.print.PrintServiceLookup.class.getDeclaredClasses()[0], null);

Answer 5

I'm not sure what you were expecting. Here's the output for my Windows Vista laptop running Java 7.

Number of servies found: 3
--> Available Printer 0: Win32 Printer : PamFax
--> Available Printer 1: Win32 Printer : Microsoft XPS Document Writer
--> Available Printer 2: Win32 Printer : CutePDF Writer
x
Number of servies found: 3
--> Available Printer 0: Win32 Printer : PamFax
--> Available Printer 1: Win32 Printer : Microsoft XPS Document Writer
--> Available Printer 2: Win32 Printer : CutePDF Writer

Printer services don't change every 5 minutes, even on a shared network.

Updated based on the comment:

Your Java application will have to have to be stopped and started to get the updated list of printers.

I guess I don't understand your environment. In the places I've worked, all of the printers have been defined on the network long before we ever wrote any code. Printer changes were rarely (i mean once a year) made.