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How-to-develop-Qt-in-Java.md

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How To Develop With QtJambi

Maven Central

Add qtjambi.jar to the classpath of your Java project containing the most essential Qt Core, Gui and Widgets modules. If you use Maven to build your application simply add following dependency to your project:

<dependency>
  <groupId>io.qtjambi</groupId>
  <artifactId>qtjambi</artifactId>
  <version>$VERSION</version>
</dependency>

(exchange $VERSION either by 5.15.11, 6.2.9 or by 6.4.1).

Otherwise, download QtJambi JAR file from Maven Central Repository. Find the list of all available QtJambi modules.

Example Program

Create a file Test.java containing the following code

import io.qt.widgets.*;
public class Test {
    public static void main(String[] args) {
        QApplication.initialize(args);
        QMessageBox.information(null, "QtJambi", "Hello World!");
        QApplication.shutdown();
    }
}

Compile the file:

javac -cp qtjambi-6.4.1.jar Test.java

Executing Example

For execution you need the platform dependent binaries of QtJambi. For instance, if you are working on Windows download the windows-x64 binaries. Additionally, you need Qt. Use the Qt installer to install Qt on your system. Make sure you are using the same Qt version and QtJambi version (e.g. 5.15 or 6.3). On Linux, you can alternatively use Qt system libraries (correct version provided). The required DLLs are to be found in the bin folder on Windows and lib folder on Linux and macOS, respectively.

When running a QtJambi application you have to make the locations of Qt and QtJambi libraries known to Java. Therefore, use the PATH environment (LD_LIBRARY_PATH on Linux, DYLD_FRAMEWORK_PATH on macOS) or the Java runtime property java.library.path.

The example program can be executed this way on Windows:

java -cp qtjambi-6.4.1.jar;qtjambi-native-windows-x64-6.4.1.jar;. -Djava.library.path=C:\Qt\6.4.0\msvc2019_64\bin Test

On Linux it looks this way:

java -cp qtjambi-6.4.1.jar:qtjambi-native-linux-x64-6.4.1.jar:. -Djava.library.path=<path to>/Qt/6.4.0/gcc_64/lib Test

On macOS you additionally need to use the start parameter -XstartOnFirstThread:

java -cp qtjambi-6.4.1.jar:qtjambi-native-macos-6.4.1.jar:. -Djava.library.path=<path to>/Qt/6.4.0/macos/lib -XstartOnFirstThread Test

Native Components

QtJambi automatically detects the required native component jars if they are located next to their Java counterparts. You can simply skip qtjambi-native-OS-VERSION.jar in your classpath (-cp). Nevertheless, qtjambi-native-OS-VERSION.jar bundles are extracted every time at program startup. By default, this is a process specific temporal directory purged after program shutdown. Alternatively, you can use Java system property io.qt.deploymentdir to let libraries to be exctacted and persist in user application data path or common directory (see below).

Further Reading

In general, you can start learning how to use Qt in Java as it is introduced for C++. There are a couple of specifics for QtJambi that are introduced here. Instead of starting your program with a java command as shown above you can deploy your application as executable as described here. Read here about creating self-exctracting bundles containing Qt library. Read more about developing applications for Android.

See QtJambi 5.15 API Reference Documentation, QtJambi 6.2 API Reference Documentation and QtJambi 6.4 API Reference Documentation

Useful Java System Properties for QtJambi

Following system properties are accepted by QtJambi. You can specify Java system properties as start argument -Dproperty=value or in Java code System.setProperty("property", "value").

Message Handling

  • io.qt.log-messages - By specifying any combination of ALL, CRITICAL, DEBUG, WARNING, FATAL, INFO and SYSTEM you can install a message handler causing exceptions to be thrown in the event of a message of given type.
  • io.qt.exceptions-for-messages - By specifying any combination of ALL, CRITICAL, DEBUG, WARNING, FATAL, INFO and SYSTEM you can install a message handler forwarding messages of given types to Java logging.

Library Management

  • io.qt.library-path-override - Use this property if you want to force Java to load Qt and QtJambi libraries from other paths than given by java.library.path.
  • io.qt.debug - Specify debug to force Java using debug libraries of Qt and QtJambi.
  • io.qt.verbose-loading - Specify true to cause QtJambi to print out library loading steps.
  • io.qt.pluginpath - Specify list of paths added as plugin search path.

If you native library bundles QtJambi extracts these components to temporary directory each time at program startup. Typically, it is a process specific directory purged at program termination.

  • io.qt.keep-temp-deployment - Specify true to avoid library deletion at program termination. The libraries remain in temporary directory instead.
  • io.qt.deploymentdir - Specify user to let QtJambi extract libraries to user's application data directory. Specify common to let them be extracted to common program data directory. Specify a target directory to let them be extracted there.
  • io.qt.no-deployment-spec - Specify true if you want to inhibit the search for native library bundles at all and load QtJambi from library path instead.

QtJambi Runtime

  • io.qt.allow-nonfinal-signals - Specify true to avoid exception to be thrown when detecting non-final signal declarations.
  • io.qt.no-library-shutdown-hook - Specify true to avoid library shutdown at program termination.
  • io.qt.disable-thread-affinity-check - Specify true to avoid thread affinity checks when accessing QObjects. Use this property to improve performance in release versions of your well tested applications.
  • io.qt.disable-event-thread-affinity-check - ...the same applying to access checks during event handling.
  • io.qt.no-app-deletion - Specify true if you combine native code with Java code and your QCoreApplication instance has been created elsewhere than inside Java.

QML

  • io.qt.enabled-qml-debugging - Specify true to allow QML debugging for the entire runtime of the application.
  • io.qt.enabled-qml-debugging-nowarn - ...also inhibits a security warning.

Along with this use program argument -qmljsdebugger=... to enable QML debugging for Qt.