Resolve circular references (p1): C++ GATTHandler, GATTService, pp are not owner of their resepctive backreference

GATTHandler, GATTService, pp are not owner of their resepctive backreference,
hence use std::weak_ptr for backreferences in general and validate on usage (nullptr, if destructed).

No DBTDevice has been ever destructed after using GATTHandler and discovering all GATT services.
In contrast to Java, C++ has no magic GC and hence shared_ptr use_count gets only increased
when emplying circular backreferences - none gets destructed.

Current ownership relationship is:
  DBTAdapter -> DBTDevice -> GATTHandler -> GATTService ...

each contains a backreference, now using a weak_ptr.

Result is that depending on the use-case, DBTDevice instances are destructed:
- Using device->remove(): Immediately
- No explicit device->remove(): Adapter keeps sharedDevices, destruction occurs at end.

3 files changed, 18 insertions, 6 deletions

diff --git a/java/jni/direct_bt/DBTDevice.cxx b/java/jni/direct_bt/DBTDevice.cxx
index 0a75b77c..20b6060b 100644
--- a/java/jni/direct_bt/DBTDevice.cxx
+++ b/java/jni/direct_bt/DBTDevice.cxx
@@ -355,8 +355,12 @@ jobject Java_direct_1bt_tinyb_DBTDevice_getServicesImpl(JNIEnv *env, jobject obj
         std::function<jobject(JNIEnv*, jclass, jmethodID, GATTService*)> ctor_service =
                 [](JNIEnv *env, jclass clazz, jmethodID clazz_ctor, GATTService *service)->jobject {
                     // prepare adapter ctor
-                    JavaGlobalObj::check(service->device->getJavaObject(), E_FILE_LINE);
-                    jobject jdevice = JavaGlobalObj::GetObject(service->device->getJavaObject());
+                    std::shared_ptr<DBTDevice> device = service->getDevice();
+                    if( nullptr == device ) {
+                        throw IllegalStateException("Service's DBTDevice destructed: "+service->toString(), E_FILE_LINE);
+                    }
+                    JavaGlobalObj::check(device->getJavaObject(), E_FILE_LINE);
+                    jobject jdevice = JavaGlobalObj::GetObject(device->getJavaObject());
                     const jboolean isPrimary = service->isPrimary;
                     const jstring uuid = from_string_to_jstring(env,
                             directBTJNISettings.getUnifyUUID128Bit() ? service->type->toUUID128String() :
diff --git a/java/jni/direct_bt/DBTGattCharacteristic.cxx b/java/jni/direct_bt/DBTGattCharacteristic.cxx
index 38a55358..fb966a93 100644
--- a/java/jni/direct_bt/DBTGattCharacteristic.cxx
+++ b/java/jni/direct_bt/DBTGattCharacteristic.cxx
@@ -76,8 +76,12 @@ jobject Java_direct_1bt_tinyb_DBTGattCharacteristic_getDescriptorsImpl(JNIEnv *e
         std::function<jobject(JNIEnv*, jclass, jmethodID, GATTDescriptor *)> ctor_desc =
                 [](JNIEnv *env, jclass clazz, jmethodID clazz_ctor, GATTDescriptor *descriptor)->jobject {
                     // prepare adapter ctor
-                    JavaGlobalObj::check(descriptor->characteristic->getJavaObject(), E_FILE_LINE);
-                    jobject jcharacteristic = JavaGlobalObj::GetObject(descriptor->characteristic->getJavaObject());
+                    std::shared_ptr<GATTCharacteristic> characteristic = descriptor->getCharacteristic();
+                    if( nullptr == characteristic ) {
+                        throw IllegalStateException("Descriptor's GATTCharacteristic destructed: "+descriptor->toString(), E_FILE_LINE);
+                    }
+                    JavaGlobalObj::check(characteristic->getJavaObject(), E_FILE_LINE);
+                    jobject jcharacteristic = JavaGlobalObj::GetObject(characteristic->getJavaObject());
 
                     const jstring uuid = from_string_to_jstring(env,
                             directBTJNISettings.getUnifyUUID128Bit() ? descriptor->type->toUUID128String() :
diff --git a/java/jni/direct_bt/DBTGattService.cxx b/java/jni/direct_bt/DBTGattService.cxx
index 4ded7658..f00f9e2f 100644
--- a/java/jni/direct_bt/DBTGattService.cxx
+++ b/java/jni/direct_bt/DBTGattService.cxx
@@ -76,8 +76,12 @@ jobject Java_direct_1bt_tinyb_DBTGattService_getCharacteristicsImpl(JNIEnv *env,
         std::function<jobject(JNIEnv*, jclass, jmethodID, GATTCharacteristic *)> ctor_char =
                 [](JNIEnv *env, jclass clazz, jmethodID clazz_ctor, GATTCharacteristic *characteristic)->jobject {
                     // prepare adapter ctor
-                    JavaGlobalObj::check(characteristic->service->getJavaObject(), E_FILE_LINE);
-                    jobject jservice = JavaGlobalObj::GetObject(characteristic->service->getJavaObject());
+                    std::shared_ptr<GATTService> service = characteristic->getService();
+                    if( nullptr == service ) {
+                        throw IllegalStateException("Characteristic's GATTService destructed: "+characteristic->toString(), E_FILE_LINE);
+                    }
+                    JavaGlobalObj::check(service->getJavaObject(), E_FILE_LINE);
+                    jobject jservice = JavaGlobalObj::GetObject(service->getJavaObject());
 
                     std::vector<std::unique_ptr<std::string>> props = GATTCharacteristic::getPropertiesStringList(characteristic->properties);
                     unsigned int props_size = props.size();