qjnienvironment.cpp

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// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
#include "qjnienvironment.h"
#include "qjnihelpers_p.h"
 
#include <QtCore/QThread>
#include <QtCore/QThreadStorage>
 
QT_BEGIN_NAMESPACE
 
/*!
    \class QJniEnvironment
    \inmodule QtCore
    \since 6.1
    \brief The QJniEnvironment class provides access to the JNI Environment (JNIEnv).
 
    When using JNI, the \l {JNI tips: JavaVM and JNIEnv}{JNIEnv} class is a pointer to a function
    table and a member function for each JNI function that indirects through the table. \c JNIEnv
    provides most of the JNI functions. Every C++ native function receives a \c JNIEnv as the first
    argument. The JNI environment cannot be shared between threads.
 
    Since \c JNIEnv doesn't do much error checking, such as exception checking and clearing,
    QJniEnvironment allows you to do that easily.
 
    For more information about JNIEnv, see \l {Java: Interface Function Table}.
 
    \note This API has been designed and tested for use with Android.
    It has not been tested for other platforms.
*/
 
class QJniEnvironmentPrivate
{
public:
    JNIEnv *jniEnv = nullptr;
};
 
class QJniEnvironmentPrivateTLS
{
public:
    inline ~QJniEnvironmentPrivateTLS()
    {
        QtAndroidPrivate::javaVM()->DetachCurrentThread();
    }
};
 
Q_GLOBAL_STATIC(QThreadStorage<QJniEnvironmentPrivateTLS *>, jniEnvTLS)
 
 
/*!
    Constructs a new JNI Environment object and attaches the current thread to the Java VM.
*/
QJniEnvironment::QJniEnvironment()
    : d(new QJniEnvironmentPrivate{})
{
    d->jniEnv = getJniEnv();
}
 
/*!
    Returns the JNIEnv pointer for the current thread.
 
    The current thread will be attached to the Java VM.
*/
JNIEnv *QJniEnvironment::getJniEnv()
{
    JNIEnv *jniEnv = nullptr;
 
    JavaVM *vm = QtAndroidPrivate::javaVM();
    const jint ret = vm->GetEnv((void**)&jniEnv, JNI_VERSION_1_6);
 
    if (ret == JNI_EDETACHED) { // We need to (re-)attach
        const QByteArray threadName = QThread::currentThread()->objectName().toUtf8();
        JavaVMAttachArgs args = { JNI_VERSION_1_6,
                                  threadName.isEmpty() ? "QtThread" : threadName.constData(),
                                  nullptr
                                };
        if (vm->AttachCurrentThread(&jniEnv, &args) == JNI_OK) {
            if (!jniEnvTLS->hasLocalData()) // If we attached the thread we own it.
                jniEnvTLS->setLocalData(new QJniEnvironmentPrivateTLS);
        }
    }
    return jniEnv;
}
 
/*!
    \fn QJniEnvironment::~QJniEnvironment()
 
    Detaches the current thread from the Java VM and destroys the QJniEnvironment object.
    This will clear any pending exception by calling checkAndClearExceptions().
*/
QJniEnvironment::~QJniEnvironment()
{
    checkAndClearExceptions();
}
 
/*!
    Returns \c true if this instance holds a valid JNIEnv object.
 
    \since 6.2
*/
bool QJniEnvironment::isValid() const
{
    return d->jniEnv;
}
 
/*!
    Provides access to the JNI Environment's \c JNIEnv pointer.
*/
JNIEnv *QJniEnvironment::operator->() const
{
    return d->jniEnv;
}
 
/*!
    Returns the JNI Environment's \c JNIEnv object.
*/
JNIEnv &QJniEnvironment::operator*() const
{
    return *d->jniEnv;
}
 
/*!
    Returns the JNI Environment's \c JNIEnv pointer.
*/
JNIEnv *QJniEnvironment::jniEnv() const
{
    return d->jniEnv;
}
 
/*!
    Searches for \a 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 \a className is not found.
 
    A use case for this function is searching for a class to call a JNI method
    that takes a \c 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 \c CustomClass and then calls the
    \c printFromJava() method:
 
    \code
    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>());
    \endcode
 
    \note This call returns a global reference to the class object from the
    internally cached classes.
*/
jclass QJniEnvironment::findClass(const char *className)
{
    return QtAndroidPrivate::findClass(className, d->jniEnv);
}
 
/*!
    Searches for an instance method of a class \a clazz. The method is specified
    by its \a methodName and \a signature.
 
    Returns the method ID or \c 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.
 
    \since 6.2
*/
jmethodID QJniEnvironment::findMethod(jclass clazz, const char *methodName, const char *signature)
{
    if (clazz) {
        jmethodID id = d->jniEnv->GetMethodID(clazz, methodName, signature);
        if (!checkAndClearExceptions(d->jniEnv))
            return id;
    }
 
    return nullptr;
}
 
/*!
    \fn template<typename ...Args> jmethodId QJniEnvironment::findMethod(jclass clazz, const char *methodName)
    \since 6.4
 
    Searches for an instance method of a class \a clazz. The method is specified
    by its \a methodName, the signature is deduced from the template parameters.
 
    Returns the method ID or \c nullptr if the method is not found.
*/
 
/*!
    Searches for a static method of a class \a clazz. The method is specified
    by its \a methodName and \a signature.
 
    Returns the method ID or \c 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.
 
    \code
    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>());
    \endcode
 
    \since 6.2
*/
jmethodID QJniEnvironment::findStaticMethod(jclass clazz, const char *methodName, const char *signature)
{
    if (clazz) {
        jmethodID id = d->jniEnv->GetStaticMethodID(clazz, methodName, signature);
        if (!checkAndClearExceptions(d->jniEnv))
            return id;
    }
 
    return nullptr;
}
 
/*!
    \fn template<typename ...Args> jmethodId QJniEnvironment::findStaticMethod(jclass clazz, const char *methodName)
    \since 6.4
 
    Searches for an instance method of a class \a clazz. The method is specified
    by its \a methodName, the signature is deduced from the template parameters.
 
    Returns the method ID or \c nullptr if the method is not found.
 
    \code
    QJniEnvironment env;
    jclass javaClass = env.findClass("org/qtproject/example/android/CustomClass");
    jmethodID methodId = env.findStaticMethod<void, jstring>(javaClass, "staticJavaMethod");
    QJniObject javaMessage = QJniObject::fromString("findStaticMethod example");
    QJniObject::callStaticMethod<void>(javaClass,
                                       methodId,
                                       javaMessage.object<jstring>());
    \endcode
*/
 
/*!
    Searches for a member field of a class \a clazz. The field is specified
    by its \a fieldName and \a signature.
 
    Returns the field ID or \c 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.
 
    \since 6.2
*/
jfieldID QJniEnvironment::findField(jclass clazz, const char *fieldName, const char *signature)
{
    if (clazz) {
        jfieldID id = d->jniEnv->GetFieldID(clazz, fieldName, signature);
        if (!checkAndClearExceptions())
            return id;
    }
 
    return nullptr;
}
 
/*!
    \fn template<typename T> jfieldID QJniEnvironment::findField(jclass clazz, const char *fieldName)
    \since 6.4
 
    Searches for a member field of a class \a clazz. The field is specified
    by its \a fieldName. The signature of the field is deduced from the template parameter.
 
    Returns the field ID or \c nullptr if the field is not found.
*/
 
/*!
    Searches for a static field of a class \a clazz. The field is specified
    by its \a fieldName and \a signature.
 
    Returns the field ID or \c 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.
 
    \since 6.2
*/
jfieldID QJniEnvironment::findStaticField(jclass clazz, const char *fieldName, const char *signature)
{
    if (clazz) {
        jfieldID id = d->jniEnv->GetStaticFieldID(clazz, fieldName, signature);
        if (!checkAndClearExceptions())
            return id;
    }
 
    return nullptr;
}
 
/*!
    \fn template<typename T> jfieldID QJniEnvironment::findStaticField(jclass clazz, const char *fieldName)
    \since 6.4
 
    Searches for a static field of a class \a clazz. The field is specified
    by its \a fieldName. The signature of the field is deduced from the template parameter.
 
    Returns the field ID or \c nullptr if the field is not found.
*/
 
/*!
    Returns the Java VM interface for the current process. Although it might
    be possible to have multiple Java VMs per process, Android allows only one.
 
*/
JavaVM *QJniEnvironment::javaVM()
{
    return QtAndroidPrivate::javaVM();
}
 
/*!
    \fn template <typename Class> bool QJniEnvironment::registerNativeMethods(std::initializer_list<JNINativeMethod> methods)
    \overload
 
    Registers the Java methods in \a methods with the Java class represented by
    \c Class, and returns whether the registration was successful.
 
    The \c Class type has to be declared within the QtJniTypes namespace using
    the Q_DECLARE_JNI_CLASS macro. Functions that are implemented as free C or
    C++ functions have to be declared using one of the
    Q_DECLARE_JNI_NATIVE_METHOD macros, and passed into the registration using
    the Q_JNI_NATIVE_METHOD macro.
 
    \include jni.qdoc register-free-function
 
    For functions that are implemented as static class member functions, use
    the \l{Q_DECLARE_JNI_NATIVE_METHOD_IN_CURRENT_SCOPE}{macros for scoped
    functions} instead.
 
    \include jni.qdoc register-scoped-function
*/
 
/*!
    \overload
 
    Registers the Java methods in the array \a methods of size \a size, each of
    which can call native C++ functions from class \a className. These methods
    must be registered before any attempt to call them.
 
    Returns \c true if the registration is successful, otherwise \c false.
 
    Each element in the methods array consists of:
    \list
        \li The Java method name
        \li Method signature
        \li The C++ functions that will be executed
    \endlist
 
    \code
    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);
    \endcode
*/
bool QJniEnvironment::registerNativeMethods(const char *className, const JNINativeMethod methods[],
                                            int size)
{
    const jclass clazz = findClass(className);
 
    if (!clazz)
        return false;
 
    return registerNativeMethods(clazz, methods, size);
}
 
/*!
    \fn bool QJniEnvironment::registerNativeMethods(const char *className, std::initializer_list<JNINativeMethod> methods)
    \overload
 
    Registers the native functions methods in \a methods for the Java class \a className.
    Returns \c true if the registration is successful, otherwise \c false.
*/
 
#if QT_DEPRECATED_SINCE(6, 2)
/*!
    \overload
    \deprecated [6.2] Use the overload with a const JNINativeMethod[] instead.
 
    Registers the Java methods in the array \a methods of size \a size, each of
    which can call native C++ functions from class \a className. These methods
    must be registered before any attempt to call them.
 
    Returns \c true if the registration is successful, otherwise \c false.
 
    Each element in the methods array consists of:
    \list
        \li The Java method name
        \li Method signature
        \li The C++ functions that will be executed
    \endlist
 
    \code
    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);
    \endcode
*/
bool QJniEnvironment::registerNativeMethods(const char *className, JNINativeMethod methods[],
                                            int size)
{
    return registerNativeMethods(className, const_cast<const JNINativeMethod*>(methods), size);
}
#endif
/*!
    \overload
 
    This overload uses a previously cached jclass instance \a clazz.
 
    \code
    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);
    \endcode
*/
bool QJniEnvironment::registerNativeMethods(jclass clazz, const JNINativeMethod methods[],
                                            int size)
{
    if (d->jniEnv->RegisterNatives(clazz, methods, size) < 0) {
        checkAndClearExceptions();
        return false;
    }
    return true;
}
 
/*!
    \fn bool QJniEnvironment::registerNativeMethods(jclass clazz, std::initializer_list<JNINativeMethod> methods)
    \overload
 
    Registers the native functions methods in \a methods for the Java class \a clazz.
    Returns \c true if the registration is successful, otherwise \c false.
*/
 
/*!
    \enum QJniEnvironment::OutputMode
 
    \value Silent The exceptions are cleaned silently
    \value Verbose Prints the exceptions and their stack backtrace as an error
           to \c stderr stream.
*/
 
/*!
    \fn QJniEnvironment::checkAndClearExceptions(OutputMode outputMode = OutputMode::Verbose)
 
    Cleans any pending exceptions either silently or reporting stack backtrace,
    depending on the \a outputMode.
 
    In contrast to \l QJniObject, which handles exceptions internally, if you
    make JNI calls directly via \c JNIEnv, you need to clear any potential
    exceptions after the call using this function. For more information about
    \c JNIEnv calls that can throw an exception, see \l {Java: JNI Functions}{JNI Functions}.
 
    \return \c true when a pending exception was cleared.
*/
bool QJniEnvironment::checkAndClearExceptions(QJniEnvironment::OutputMode outputMode)
{
    return checkAndClearExceptions(d->jniEnv, outputMode);
}
 
namespace {
    // Any pending exception need to be cleared before calling this
    QString exceptionMessage(JNIEnv *env, jthrowable exception)
    {
        if (!exception)
            return {};
 
        auto logError = []() {
            qWarning() << "QJniEnvironment: a null object returned or an exception occurred while "
                          "fetching a prior exception message";
        };
 
        auto checkAndClear = [env]() {
            if (Q_UNLIKELY(env->ExceptionCheck())) {
                env->ExceptionClear();
                return true;
            }
            return false;
        };
 
        const jclass logClazz = env->FindClass("android/util/Log");
        if (checkAndClear() || !logClazz) {
            logError();
            return {};
        }
 
        const jmethodID methodId = env->GetStaticMethodID(logClazz, "getStackTraceString",
                                            "(Ljava/lang/Throwable;)Ljava/lang/String;");
        if (checkAndClear() || !methodId) {
            logError();
            return {};
        }
 
        jvalue value;
        value.l = static_cast<jobject>(exception);
        const jobject messageObj = env->CallStaticObjectMethodA(logClazz, methodId, &value);
        const jstring jmessage = static_cast<jstring>(messageObj);
        if (checkAndClear())
            return {};
 
        char const *utfMessage = env->GetStringUTFChars(jmessage, 0);
        const QString message = QString::fromUtf8(utfMessage);
 
        env->ReleaseStringUTFChars(jmessage, utfMessage);
 
        return message;
    }
} // end namespace
 
/*!
    Cleans any pending exceptions for \a env, either silently or reporting
    stack backtrace, depending on the \a outputMode. This is useful when you
    already have a \c JNIEnv pointer such as in a native function implementation.
 
    In contrast to \l QJniObject, which handles exceptions internally, if you
    make JNI calls directly via \c JNIEnv, you need to clear any potential
    exceptions after the call using this function. For more information about
    \c JNIEnv calls that can throw an exception, see \l {Java: JNI Functions}{JNI Functions}.
 
    \return \c true when a pending exception was cleared.
*/
bool QJniEnvironment::checkAndClearExceptions(JNIEnv *env, QJniEnvironment::OutputMode outputMode)
{
    if (Q_UNLIKELY(env->ExceptionCheck())) {
        if (outputMode == OutputMode::Verbose) {
            if (jthrowable exception = env->ExceptionOccurred()) {
                env->ExceptionClear();
                const QString message = exceptionMessage(env, exception);
                // Print to QWARN since env->ExceptionDescribe() does the same
                if (!message.isEmpty())
                    qWarning().noquote() << message;
                env->DeleteLocalRef(exception);
            } else {
                // if the exception object is null for some reason just
                env->ExceptionDescribe();
                env->ExceptionClear();
            }
        } else {
            env->ExceptionClear();
        }
 
        return true;
    }
 
    return false;
}
 
QT_END_NAMESPACE