When compiled in our platform, we must include the code in a new shared library into the file example java HelloJavaJNI. The point of contention or argument supplied into our method is system. However, the entire path to the location hosting the library we just produced must be added, telling Java where to find our native libraries, shown below:.
It is lovely to say hello, but it is not particularly beneficial. Some parameters must be included in the foreign method. We will make another new class named ParameterIllustrationJNI with two foreign methods that uses distinct types of parameters and returns them. After that, we will need to use javac-h to produce another. After that, we must construct an a.
In our illustration above, we have utilized these methods given through these JNI environment instances. Looking at this final illustration, we will explore how to incorporate Java objects within native source codes. Suppose we begin by making a new class and name it UtilizeInformation ; we will store user information:. Next, we will have to create another java class. This time, the class will manage objects of the class used. We have just created a java class that contained a native method or function in the code above.
The function manages our objects in the java class we have just created. In the final step, we will construct the. Programmers use the JNI to write native methods to handle those situations when an application cannot be written entirely in the Java programming language.
For example, you may need to use native methods and the JNI in the following situations: The standard Java class library may not support the platform-dependent features needed by your application. You may already have a library or application written in another programming language and you wish to make it accessible to Java applications.
You may want to implement a small portion of time-critical code in a lower-level programming language, such as assembly, and then have your Java application call these functions.
Programming through the JNI framework lets you use native methods to do many operations. Native methods may represent legacy applications or they may be written explicitly to solve a problem that is best handled outside of the Java programming environment. Invoke the native method from Java For example, our Java code could look like this:. The library filename will be called libHelloImpl. We use the JDK javah utility to generate the header file Hello.
We then create Hello. Compiling We are now ready to compile our program and run it. The actions performed by the runtime depend upon whether it returned a pointer to the actual data or a copy of it:.
For example, you might want to get an array, modify it in place, pass pieces to other functions, and then discard the changes. If you know that JNI is making a new copy for you, there's no need to create another "editable" copy. If JNI is passing you the original, then you do need to make your own copy.
This is not the case. If no copy buffer was allocated, then the original memory must be pinned down and can't be moved by the garbage collector. Consider the following:. This grabs the array, copies the first len byte elements out of it, and then releases the array. Depending upon the implementation, the Get call will either pin or copy the array contents. Exceptions You must not call most JNI functions while an exception is pending. Your code is expected to notice the exception via the function's return value, ExceptionCheck , or ExceptionOccurred and return, or clear the exception and handle it.
Many JNI calls can throw an exception, but often provide a simpler way of checking for failure. However, if you call a method using a function like CallObjectMethod , you must always check for an exception, because the return value is not going to be valid if an exception was thrown.
Upon returning to managed from native code, the exception will be noted and handled appropriately. Native code can "catch" an exception by calling ExceptionCheck or ExceptionOccurred , and clear it with ExceptionClear. As usual, discarding exceptions without handling them can lead to problems. JNI does very little error checking. Errors usually result in a crash. Android also offers a mode called CheckJNI, where the JavaVM and JNIEnv function table pointers are switched to tables of functions that perform an extended series of checks before calling the standard implementation.
Accessibility of methods and fields is still not checked: access restrictions don't apply to native code. If you have a rooted device, you can use the following sequence of commands to restart the runtime with CheckJNI enabled:. Change the property to any other value or simply rebooting will disable CheckJNI again. You can also set the android:debuggable attribute in your application's manifest to turn on CheckJNI just for your app.
Note that the Android build tools will do this automatically for certain build types. Native libraries You can load native code from shared libraries with the standard System. In practice, older versions of Android had bugs in PackageManager that caused installation and update of native libraries to be unreliable. The ReLinker project offers workarounds for this and other native library loading problems. Call System. The argument is the "undecorated" library name, so to load libfubar.
If you have only one class with native methods, it makes sense for the call to System. Otherwise you might want to make the call from Application so you know that the library is always loaded, and always loaded early.
There are two ways that the runtime can find your native methods. You can either explicitly register them with RegisterNatives , or you can let the runtime look them up dynamically with dlsym. The advantage of letting the runtime discover your functions is that it's slightly less code to write. To instead use "discovery" of native methods, you need to name them in a specific way see the JNI spec for details.
This means that if a method signature is wrong, you won't know about it until the first time the method is actually invoked.
When called from other contexts, FindClass uses the class loader associated with the method at the top of the Java stack, or if there isn't one because the call is from a native thread that was just attached it uses the "system" class loader.
The system class loader does not know about your application's classes, so you won't be able to look up your own classes with FindClass in that context. To support architectures that use bit pointers, use a long field rather than an int when storing a pointer to a native structure in a Java field. Until Android 2. Working around this requires using explicit registration or moving the native methods out of inner classes.
Detaching threads Until Android 2. The runtime also uses a pthread key destructor function, so it'd be a race to see which gets called first.
Weak global references Until Android 2. Older versions will vigorously reject attempts to use them.
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