QJniEnvironment Class

Searches for className using all available class loaders. Qt on Android uses a custom class loader to load all the .jar files and it must be used to find any classes that are created by that class loader because these classes are not visible when using the default class loader.

Returns the class pointer or null if className is not found.

A use case for this function is searching for a class to call a JNI method that takes a jclass. This can be useful when doing multiple JNI calls on the same class object which can a bit faster than using a class name in each call. Additionally, this call looks for internally cached classes first before doing a JNI call, and returns such a class if found. The following code snippet creates an instance of the class CustomClass and then calls the printFromJava() method:

QJniEnvironment env;
jclass javaClass = env.findClass("org/qtproject/example/android/CustomClass");
QJniObject javaMessage = QJniObject::fromString("findClass example");
QJniObject::callStaticMethod<void>(javaClass, "printFromJava",
                                   "(Ljava/lang/String;)V", javaMessage.object<jstring>());

Searches for a member field of a class clazz. The field is specified by its fieldName and signature.

Returns the field ID or nullptr if the field is not found.

A usecase for this method is searching for class fields and caching their IDs, so that they could later be used for getting/setting the fields.

This function was introduced in Qt 6.2.

Searches for an instance method of a class clazz. The method is specified by its methodName and signature.

Returns the method ID or nullptr if the method is not found.

A usecase for this method is searching for class methods and caching their IDs, so that they could later be used for calling the methods.

This function was introduced in Qt 6.2.

Searches for a static field of a class clazz. The field is specified by its fieldName and signature.

Returns the field ID or nullptr if the field is not found.

A usecase for this method is searching for class fields and caching their IDs, so that they could later be used for getting/setting the fields.

This function was introduced in Qt 6.2.

Searches for a static method of a class clazz. The method is specified by its methodName and signature.

Returns the method ID or nullptr if the method is not found.

A usecase for this method is searching for class methods and caching their IDs, so that they could later be used for calling the methods.

QJniEnvironment env;
jclass javaClass = env.findClass("org/qtproject/example/android/CustomClass");
jmethodID methodId = env.findStaticMethod(javaClass,
                                          "staticJavaMethod",
                                          "(Ljava/lang/String;)V");
QJniObject javaMessage = QJniObject::fromString("findStaticMethod example");
QJniObject::callStaticMethod<void>(javaClass,
                                   methodId,
                                   javaMessage.object<jstring>());

This function was introduced in Qt 6.2.

Returns true if this instance holds a valid JNIEnv object.

This function was introduced in Qt 6.2.

Registers the Java methods in the array methods of size size, each of which can call native C++ functions from class className. These methods must be registered before any attempt to call them.

Returns true if the registration is successful, otherwise false.

Each element in the methods array consists of:

• The Java method name

• Method signature

• The C++ functions that will be executed

const JNINativeMethod methods[] =
                        {{"callNativeOne", "(I)V", reinterpret_cast<void *>(fromJavaOne)},
                        {"callNativeTwo", "(I)V", reinterpret_cast<void *>(fromJavaTwo)}};
QJniEnvironment env;
env.registerNativeMethods("org/qtproject/android/TestJavaClass", methods, 2);

This is an overloaded function.

This overload uses a previously cached jclass instance clazz.

JNINativeMethod methods[] {{"callNativeOne", "(I)V", reinterpret_cast<void *>(fromJavaOne)},
                           {"callNativeTwo", "(I)V", reinterpret_cast<void *>(fromJavaTwo)}};
QJniEnvironment env;
jclass clazz = env.findClass("org/qtproject/android/TestJavaClass");
env.registerNativeMethods(clazz, methods, 2);

bool QJniEnvironment::registerNativeMethods(const char *className, JNINativeMethod[] methods, int size)▲

JNINativeMethod methods[] = {{"callNativeOne", "(I)V", reinterpret_cast<void *>(fromJavaOne)},
                             {"callNativeTwo", "(I)V", reinterpret_cast<void *>(fromJavaTwo)}};
QJniEnvironment env;
env.registerNativeMethods("org/qtproject/android/TestJavaClass", methods, 2);