Jni thread attach

Jni thread attach. Depends on your system. Let's call the installation directory <JDK-DIR>. If coding this directly is awkward, in Android 2. Then, JNI_Onload() calls some Juce internal things that starts the app by calling MyJuceApp, where things fail. I can see this by running DDMS memory analyser. I tried to create a callback from jni to java, But when ever execute my code from cpp thread, findClass method return null, But the same class path & method properly executing from jni thread (Native Thread). Normally, when making a native executable program, we can choose to use static or shared libs: Aug 21, 2020 · Note that this is for the case where you don't know whether the thread already is attached. So, you can then go on to AttachCurrentThread () only if necessary, and you can record whether you had to attach, so you can later decide whether to detach. Apr 28, 2017 · attaching thread to JVM ("The JNI interface pointer (JNIEnv) is valid only in the current thread. There is logic flow at the bottom, Maybe helpful. so); We would like to show you a description here but the site won’t allow us. JNI_EEXIST is returned if the transport only supports a single environment, and the environment pointer was previously returned by the first call to the onload function. You can vote up the ones you like or vote down the ones you don't like, and go to the original project or source file by following the links above each example. section). Sep 4, 2011 · It seems to be true; the solution is to keep a persistent, "reusable" thread around - I used Executors. Both used the same version. detach_current_thread to note that it is never appropriate to use for scoped attaches; and might rarely be for permanent attaches. Thread from Java API. Normally, when interacting with a virtual machine, you attach a thread once and May 11, 2018 · Unity Technologies. A working C++ toolchain. If your code crashes, it might be because of the very fact that AttachCurrentThread is not a mutex. /* Remember that you can always get method signature using javap tool. [ −] pub struct AttachGuard<'local> { /* private fields */ } A RAII implementation of scoped guard which detaches the current thread when dropped. I think the best solution is to implement the schwartz counter as normal, but implement the ThisThread class in terms of a thread_local static Impl. 13 brings major improvements in thread management, allowing to attach the native threads permanently and safely; Executor for extra convenience and safety; and other improvements and fixes. Java does not change the thread priority on transitions back to the Java virtual machine. This is the way I'm getting JNIEnv pointers: Examples at hotexamples. Here comes a tricky bit. It doesn't do anything other then attach and then detach the worker thread. To use JNIEnv*, a thread must have been attached to the VM (using AttachCurrentThread). 3 Native multithreading and JNI. Jan 28, 2018 · I have a thread pool executing java methods from JNI side. Apr 8, 2016 · Some of these callbacks occur on threads created within the native library. Jan 8, 2024 · JNI header file – this header file for C/C++ (include/jni. What is the best way to do this? I think since the threads call the callback method, the callbacks aren't attached to the JVM, so I have to attach it? Jul 8, 2020 · You can try to clean all the background application before your debug you application in this device, it this issue will occur fewer. Sep 11, 2012 · On the C# side if I call the Java function on the main Unity thread then I get the number 3 returned: Now if I call the same Java function from a thread I created in C# then I get the number 0 returned: AndroidJNI. Rust bindings to the Java Native Interface — JNI. A native thread must «attach» itself to be able to call Java methods outside of a native Java method. posted 17 years ago. newSingleThreadExecutor() - and to call the native code only from that thread. Apr 16, 2010 · 2. And vice versa when native code calls a java method then the the java method is executed in the same thread as the native code calling the method. this code will not work correctly: void f1 () {. If you need a performance critical thread to communicate with the jvm you will need to do it with some kind of asynchronous messaging. It works. Native Methods: the JVM Meets Compiled Code. g. JNI_ENOMEM is returned if there is insufficient memory to complete initialization. Jul 13, 2020 at 7:07. Additional resources: Java Native Interface Specification (Oracle) Apr 6, 2012 · The solution to you problem can be resolved with thread_local storage (C++ 11 and higher). 21. Thread; public class Thread extends Object. It's usually best to use Thread. Jan 8, 2024 · How It Works. AndroidJavaProxy is invoked on the same thread on which the Java code calling the interface is executing. The handle 0 represents NULL. Dec 8, 2013 at 18:54. Attaching a thread as a daemon thread just means that the VM could exit without having to wait for your thread to detach first. so (if you’re on linux) or a libmylib. The only way this can work is if the C++ object is constructed and destroyed in the same thread. java public class SquareRoot {. static {. Jul 17, 2016 · 22. Toggle navigation May 19, 2015 · For this tutorial, you need to have: An installed Java Developer's Kit (JDK). To avoid unnecessary overhead, the thread really should attach itself as soon as possible (at least before the 1st JNI call), and stay attached until it no longer needs to make any further JNI calls. Using a ThreadAttacher str Feb 1, 2011 · The VM shuts down when all non-daemon threads have exited. Inside this callback method I try to create another java thread and call start on the thread. I want these callbacks to be attached to the JVM. // Static block is executed when the class is loaded. Once attached to the VM, a native thread works just like an ordinary Java thread running inside a native method. I think it fails because MyJuceApp is initialized from another thread. Call<int>("foo"); // foo == 0. Integrate the Mono object system with third-party object systems. Use Mono to host the user scripting interface, turning Mono and the scripts into a generic plugin interface. Sep 1, 2023 · When a thread is attached to JVM, it has to allocate some resources for the attached thread, even if the attached thread is done with all the JNI calls, the resources will only free up once the thread is detached before termination. The reason I am using threading is because I am sending java. tomcat. In trying to debug this issue, I have written a simple test driver which I am running on Solaris 8 with Java 1. Nov 12, 2019 · A native thread MUST attach itself to the JVM before it can make JNI calls. Thread. I have a C/C++ process which trys to create the JVM in one thread and attach to it in others. You must not pass the interface pointer from one thread to another, or cache an interface pointer and use it in multiple threads. C/C++ Compiler – we can choose between GCC, Clang, Visual Studio, or any other we like as far as it’s able to generate a native shared library for our platform. A sample example of how to implement that in C++ can be found Jun 17, 2018 · I've never worked with JNI before, but a rudimentary google search of "JNI thread safety" returned this as its first hit: The JNI interface pointer (JNIEnv *) is only valid in the current thread. If this issue is still existed, change a bigger RAM android and make a test. The JVM does not schedule threads Description. This allows you to attach to an arbitrary thread, and then it will automatically detach when the thread exist (even when you didn't create the thread and have no control over it's life cycle). Jun 12, 2012 · Is it right to set JNI interface pointer (JNIEnv* m_JavaEnv) in the constructor once and use it in the whole code ? No. Apr 22, 2019 · I'm writing a JNI wrapper around a native C++ library, but I'm having trouble attaching native threads to the JVM on Linux. May 6, 2019 · 5. Pass "destructor" reference to pthread_key_create(destKey_, threadDestructor); and inside this "destructor" call JNI method cached_jvm->DetachCurrentThread();. Jan 24, 2012 · All groups and messages Jan 8, 2024 · JNI RegisterNatives Method. See also java. Jul 10, 2021 · It looks like when JNI_Onload() is called, it's called from the Java thread. Mar 10, 2012 · 2. The JVM has two ways to find and link native methods with Java code. Embed a web application or a web service server inside your application. The Android Flutter embedding is responsible for coordinating Android OS events and app user interactions with the C/C++ engine. If you re-use the same native thread, performance will be good. Compile your code into an agent library (. 2, the Java security model allows non-system classes to load and call native methods. Posts: 3,755. Since these operations are time consuming, it is wise to store the result somewhere to be reused later on in the native side (this is caching). 3. In tutorial 2, the only conditions are 1) the code is not already initialized and 2) the main loop is running. Another way is to use the JNI RegisterNatives () method. jclass cls = (*env)->FindClass(env, "java/lang/Thread"); // Then, we can look for it's static method 'currentThread'. May 23, 2014 · This snippet will get you the env from JVM: (place this such that every instance has local copy of env and global copy of jvm) After creating the instance you have to attach these env to the current thread (i. This function is used to create a new global reference to a Java object. Aurimas-Cernius, Feb 18, 2021. JNIEXPORT void JNICALL Java_com_myapp_CApi_setFoo (JNIEnv *env, jobject thiz, jstring foo) { const char * fooStr = env->GetStringUTFChars (foo, 0); env->MonitorEnter (thiz); // same effect In JDK release 1. I tried runing with both pthreads and lwp threads with no success. If these two are met, the Java onGStreamerInitialized() method is called via the CallVoidMethod () JNI call. Nov 2, 2023 · Threads attached through JNI must call DetachCurrentThread() before they exit. The attached JNIEnv can be accessed through this guard via its Deref implementation. :warning: If your code attaches native threads — make sure to check the updated documentation of JavaVM to learn about the new features! Added Aug 12, 2002 · JNI and threads. so may be there is something android specific that I am not aware of. Android apps don't really "exit", so it's sort of a non-issue. jar". Here is a sample code to do that: // Global variable. See full list on docs. Is creating threads from JNI call backs a problem ? java. After attaching, the thread can change the thread priority by using a native method interface (for example, a POSIX API). Jul 21, 2017 · The classpath passed to addPath method from C++ should not include "-Djava. So thats the flow of JNI. Either the thread is creating natively and then attached or directly created as java. Apr 13, 2011 · I call a dll from Java using JNI. An installed Java Runtime environment (JRE). However this java thread never gets run. I have a JNI implementation in C wherein a C function attach to current JVM thread and callback a Java method, which is actually crashing the JVM upon attachCurrentThread () function call. . JNIEnv *env; The solution that I'm going with is caching the JavaVM pointer when I catch it in the JNI_OnLoad method is invoked when loading the library. The call to load the library should then be: System. Call java method. Boost this thread! It may seem like a A Frida based tool to trace use of the JNI API in Android apps. Its like it never exists. 1k 22 104 230. It has consequence - a native function 108. However, there is a way to load your code in the context of different JVM process using HotSpot Dynamic Attach API. Contribute to july1992/jniThread development by creating an account on GitHub. So the difference from JNI point of view was not between main thread vs. It's good practice to get JavaVM once in this method and store it somewhere, so you can get JNIEnv pointer from it Mar 9, 2016 · 1. Now, if you run cargo build from inside the crate directory, you should see a libmylib. My alternative solution uses boost::thread_specific_ptr to detach each native thread just May 15, 2018 · This is still a very good article, even though it was written many years ago. This library provides two modes of attachment, each ensuring the thread is promptly detached: A scoped attachment with attach_current_thread. dylib (if you are on OSX) in the target/debug directory. The test driver creates threads that either create the JVM, if not previously created, or try to attach to the JVM Sep 4, 2007 · Suppose a DllMain function is running and it creates a thread. That is, you can enter the same monitor that a java synchronized block would from java code. AndroidJNI . For most Unity APIs this means that you probably want to store the callback info somewhere and have a script to to pick that up and do calls into Unity. JavaVM *g_jvm = NULL; //Get g_jvm from jni main thread use env->GetJavaVM(&g_jvm); jobject g_obj = NULL; //Where the java function exist. 20. The JNIEnv NewGlobalRef function is a part of the Java Native Interface (JNI) library in C++. :: AttachGuard. #include <thread>. And the answer is The GetEnv () function in the JNI will return a JNIEnv if the thread is already attached to the JVM, or NULL if it isn't. The DLL calls another thirdparty library which spawns a bunch of threads and sends callbacks to my dll. 56 Java thread leaks when calling back from native thread via JNI Attaching Native Threads. For example. com The JNI interface pointer (JNIEnv) is valid only in the current thread. The most importtant is: How to async call java method and callback native. 1. Dec 30, 2022 · 0. In the native code I have to check the value of a variable from Java and for this I created a new native thread. However, doing an Attach and Detach for every callback can be a bit wasteful, depending on how often these callbacks occur. Oct 26, 2019 · To change the version you use in the command prompt you must change the path environment variable to contain the path to the desired version. oracle. Thread Apr 18, 2020 · The JNI interface pointer (JNIEnv) is valid only in the current thread. Instead, JNI provides word-sized object handles that can be passed to JNI functions to access objects indirectly. Object; org. #include <mutex>. Dec 20, 2011 · Generally speaking, the main performance cost is the thread creation at OS level. #include <iostream>. You can safely store that pointer as a global variable. apache. Oct 20, 2016 · It seems that the problem is related to this Chromium bug. std::mutex emit_mutex; Sep 13, 2014 · In android I use pthread_create to create a native thread , then in the Callback procedure ,call FindClass to get a Java class. If I use the JNI attach function I get a signal 11 core dump; if I use the getenv function it simply fails. Jan 25, 2012 · Actually, the implementation of a self-detaching thread is quite easy. source ·. My problem is that calling JavaVM^. By the way, do not create dozens of native threads. Nov 19, 2015 · Suggest to take a look on AttachCurrentThread. My implementation is based on the answer given in the link Keeping a global reference to the JNIEnv environment. Joined: Jul 31, 2013. flutter. JNI_EVERSION is returned if the version in the version argument is not JDWPTRANSPORT_VERSION_1_0. The last thing we need to do is to define our exported method. embedding. lang. Since DllMain functions are serialized, the attempt to send out the DLL_THREAD_ATTACH notifications must wait until the current Jan 14, 2021 · FindClass from any thread in Android JNI; Call static Java method from separate thread using JNI; Calling java method from different thread in ndk; My goal is to record AudioData from Oboe and send the data to Java on a different thread. path" and it should be just complete path to the . If you're 100% sure that the thread isn't already attached, then you can skip GetEnv. The JNI way of locking a mutex is MonitorEnter / MonitorExit. (some activity) //Get env in thread function and attach the env. Feb 29, 2024 · Step 1: Write the Java Code. io. : gdb, TotalView, lldb). The first one is to call a native function in a specific way so that the JVM can find it. h into the JDK directory) includes all definitions of JNI elements that we may use into our native programs. 1, this function loads a locally-defined class. jnitrace works as a dynamic analysis tracing tool similar to frida-trace or strace Jul 26, 2016 · 1. Everything works fine on Windows, but when I call AttachNativeThread() on an Ubuntu machine, it always returns -1. Instead to attach/detach the thread for every method execution i attach the thread once and on thread termination it is executed detaching. For example: Receive message A “backup”, Then call the java plugin with MsgA, The plugin porcess this event need 10second, And call 5 time native method for what ever it need. AttachCurrentThread(); int foo = javaObject. Calling attach multiple times doesnt directly cause problems but if. So I your example whenever the GetEnv call fails call AttachCurrentThread() and you should be fine. Process java method returned value. You've declared it, but never assigned. The thread will automatically detach itself once the returned guard is Oct 21, 2014 · Trying to attach a thread that is already attached is a no-op. . 2. You need to call AttachCurrentThread() for each native thread at least once before you can use any of the JNI functions. I modify it for my project like this [edit] Apr 25, 2012 · The JNI layer is a 3rd party so I can't inspect it, but at some point it creates a new thread and makes a callback to the java code. Please refer to the documentation to the chapter Attaching to the VM. Even more, you can find a screencast that demonstates how to debug JNI code using IntelliJ IDEA and CLion. I get tips from android jni tips I found the solution in FindClass from any thread in Android JNI. Mar 11, 2014 · It looks like the calls to attach and detach cause JAVA to leak a thread. That's how I solved it, in-case anyone else has a similar issue. The documentation says that "the JNI interface pointer is valid only in current thread", so one must attach the new thread to the JVM in order to get a new 8 C++ code examples are found related to "attach current thread". Java 与 C++ 通过 jni 创建线程，并方法互调. You can obtain a pointer to the JVM ( JavaVM*) with JNIEnv->GetJavaVM. Quoting the bug report: On recent versions of ART with CheckJNI turned on, this causes a spammy warning to be printed to logcat stating "Attempt to remove non-JNI local reference, dumping thread" with a thread dump, as apparently parameters are not supposed to be deleted, only objects returned as local references from native->java JNI Feb 20, 2015 · JNIEnv* has to be retrieved and used from the same thread and must not be used across threads. That's what Attach/DetachCurrentThread are for. AttachCurrentThread (); //call jni function. Apr 7, 2017 · Don't attach or detach performance critical threads to the jvm. As per Pax you should set the library path to where ever Java should look for the library. JNI functions can be used only within the process that started JVM. JavaVM* vm = _vm; const char* data = (const char *)dataArg; int isThreadAttached = attachJNIEnvToThread(vm); Under [dependencies], add jni = "0. JNI does not allow you to control other processes. I use JNI to extend my Java class with some Windows specific functions. Is your feature request related to a problem? Please describe (REQUIRED): It is important to detach a thread from the JNI to not leak any resources or lock up the system. e. For me, the main takeaway is that the pattern of Attach/Detach on every call is not optimal for Android; instead, Attach the current thread on each JNI call unconditionally (it's NOP for attached thread), and use pthread_key_create to define a destructor function that will be called before the thread exits. Local handles are only valid for the duration of a native call and only in the caller’s thread, while global handles are valid across threads and remain valid until they are destroyed explicitly. The first callback on a non JVM thread fails with a SIGSEGV which I could trace down to ignoring the negative return value (-1) of AttachCurrentThread and thus dereferencing a jenv pointer which is actually null. Tracking those calls through manual reverse engineering can be a slow and painful process. Mean while, Receive message B “echo Apr 25, 2012 · 16. Jun 29, 2018 · In line static_vm->AttachCurrentThread(&env, NULL); you try to attach current thread to JVM with null JNIEnv pointer. You have to attach to JVM where JNI code is running and debug native code using external tools (e. Feb 6, 2014 · It runs successfully on linux. May 29, 2009 · Since a C++ wrapper for the Java callbacks is needed anyway, one easy solution is to attach before each callback and detach afterwards. This function can be useful when you jni. Threads also have to be detached using DetachCurrentThread before they exit. Hi Jannik, I just checked the version used for both my Eclipse and command prompt . Hope you are doing well. ") joining threads The Sun Java(TM) Tutorial provides the following description of JNI [2]: The JNI framework lets your native method utilize Java objects in the same way that Java code uses these objects. 1" Add a new [lib] section and under it, crate_type = ["cdylib"]. jni::LocalObject always fall off scope at the conclusion of the surrounding JNI call and are valid only to a single thread, however jni::GlobalObject may be held indefinitely and are thread safe. Complete example with outputs: // header file. setDaemon(boolean on). Thread created in Java are already attached. Java provides the native keyword that’s used to indicate that the method implementation will be provided by a native code. 0 (Eclair) and higher you can use pthread_key_create() to define a destructor function that will be called before the thread exits, and call DetachCurrentThread() from there. A global reference means that the Java object will not be garbage collected until explicitly deleted in C++. Let's say the installation directory is <JRE-DIR>. Let’s start by creating a simple Java program that calculates the square root of a number using JNI. class. FlutterJNI. // SquareRoot. com: 30. 1 JNI Threads creation. Returns 0 on success; returns a negative number on failure. jar file, i. I was looking for JNI_OnLoad function in your files but didn't find it. JNI calls require a JNI Environment, which is different for every thread. // First, we have to find Thread class. jni. so. 0 (Eclair) and higher you can use pthread_key_create to define a destructor function that will be called before the thread exits, and call DetachCurrentThread from there. Aug 12, 2002 · The test driver creates threads that either create the JVM, if not previously created, or try to attach to the JVM created in a previous thread. So sequence is: 1) get already running JVM, attach current thread to the JVM, get JNI environment pointer and then call addPath java function (of another running thread) from 2. It is thread-specific. Later, in the new thread, you can either use AttachCurrentThread to attach the new thread to the JVM if you created it in C/C++ or simply GetEnv if you created the thread in java code which I do not assume since Jun 23, 2016 · The JNI interface pointer (JNIEnv) is valid only in the current thread. A thread must be attached to the VM before any other JNI calls can be made. so and loaded based on the name. loadLibrary("hello"); Linux libraries are referenced by the convention lib name . You can (as mentioned by Alex) resort to java. It seems to work. static function AttachCurrentThread () : int. A native method can create Java objects, including arrays and strings, and then inspect and use these objects to perform its tasks. EDIT 2: Changed the signature of the following: void * attachThreadToJavaVMAndPrint(void* dataArg) {. As we noted above, a kernel thread object is created and scheduled, and the first thing the thread does is notify all the DLLs with DLL_THREAD_ATTACH. jni::LocalObject is built by either wrapping a jobject passed to native JNI from Java, or constructing a new object from native (see Constructors). #include <memory>. A native function is executed in the same thread as the java method that calls the native function. "C:\\folder\\abc. I have written a really simple test app that demonstrates my issue, start test is called from JAVA and kicks off the thread. The Invocation API component of the Java Native Interface (JNI) permits a user to embed a Java virtual machine within their application. Doing so will ensure that you have sufficient stack space, that you're in the correct ThreadGroup , and that you're using the same ClassLoader as your Java code. I'm currently in the process of adding JNI functionality into a legacy delphi app. – Leon. Description. Should another thread need to access the Java VM, it must first call AttachCurrentThread() to attach itself to the VM and obtain a JNI interface pointer. Flutter's engine is built with C/C++. Attaches the current thread to a Java (Dalvik) VM. some other thread, but in using different threads in consecutive calls. Your library name should be libhello. but it does'nt work. public class FlutterJNI extends Object. Object. For making it i used TLS because the TLS destructor is executed when thread is terminated. If your method JNIGame::createEmptyState() is called by multiple threads you have to use a real mutex. JNI caching regards only these JNI function calls. In a single-threaded environment everything works fine, but as soon as I move into multi-threaded environment, things start to become hairy. See Also: Java Native Interface Specification (Oracle) Threads attached through JNI must call DetachCurrentThread before they exit. – technomage. In Eclipse you can select the used version in the settings. Contribute to jni-rs/jni-rs development by creating an account on GitHub. JNI_onLoad - get JavaVM (get jclass for calling static methods) Then need to call something from Java to C: GetEnv () to get JNIEnv and AttachCurrentThread () to use it. engine. Dec 8, 2013 · 4ntoine. Note that the JDK installation sets up a JRE automatically. Native libraries contained within Android Apps often make use of the JNI API to utilize the Android Runtime. The JVM needs to synchronise with the garbage collector and the garbage collector is one massive single threaded critical section. This happens in the swig generated code. static JavaVM * s_Jvm; JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void* reserved) {. The JNI does not create any new thread behind the scene. Dec 24, 2017 · 4. you call detach then the thread will immediately detach. Some VMs may cache the mapped native thread, so that not much happens in the attach. So, the class definitely exists, because JNI_Onload finds it. Dec 15, 2012 · JNI methods like FindClass, GetMethodID, GetFieldID are expensive operation that are guaranteed to generate the same result over the life of the JVM. // Here we load a native library named "SquareRoot". I thought that caching JNI related items was considered bad practice, but oh well. However, this may be very expensive; I haven’t measured the cost, but one post I found quoted a factor of 6 slowdown. It searches the directories and zip files specified by the CLASSPATH environment variable for the class with the specified name. Since Java 2 SDK release 1. Let me know if I missed any pros/cons of this approach and if this is a good approach to follow. JavaVM::new to note that unlike JNI counter-part, the thread remains detached and can be attached using any attach_… methods (link to apr. AttachCurrentThread. In fact, there are more Jul 6, 2023 · The low-level JNI API supports the idea that you can attach a thread to a JVM in such a way that it won't automatically get detached from the JVM when destroying the VM! The high-level idea behind Java daemon threads is that applications could use them for work that they don't necessarily want to block the application from exiting (things that Aug 6, 2021 · Skip to content. e current instance) JNIEnv * g_env; int getEnvStat = jvm->GetEnv((void **)&g_env, JNI_VERSION_1_8); A launcher that launches background threads to perform some work on behalf of the application. Hope that helps! Thanks, but as I said, these threads are often-times created and destroyed by library code that I Jul 14, 2010 · to android-ndk. When it comes to debugging JNI code, you can find some hints inside JNI Cookbook as well. Mar 4, 2018 · JNI Attach/Detach thread memory management. Interface between Flutter embedding's Java code and Flutter engine's C/C++ code. May 16, 2024 · Until a thread is attached, it has no JNIEnv, and cannot make JNI calls. AttachCurrentThread ( JavaVM, @JEnv, nil ); returns 0, but puts the JEnv pointer to 24. As the name suggests, RegisterNatives () registers native methods with the class passed as an argument. Nov 11, 2017 · 1. Overall, you'll want to reduce the number of JNI crossings; if you can skip the JNI until the very end, you'll have better performance. s_Jvm = vm; return JNI_VERSION_1_4; May 31, 2016 · The JNI interface pointer (JNIEnv) is valid only in the current thread. start() to create any thread that needs to call in to Java code. ba ix jl an xt ke xm zq gf sw