JNI Example

JNI Example

Original article: http://android.wooyd.org/JNIExample/

callback부분에서 FindClass를 이용해 native thread에서 android system call을 부르는 부분이 볼 만 하다.


JNIExample.nw


Important notice

The instructions in these document are applicable to older Android
firmwares. Starting with firmware version 1.5 the Android NDK has
been released, which makes it much easier to deal with native code.
If you are coding for firmware 1.5 or newer, it is strongly recommended
to use Android NDK instead of following this guide.


JNI Examples for Android

 

Jurij Smakov
jurij@wooyd.org

Table of Contents

 

  • Licence
  • Introduction
  • Java interface
  • Native library implementation
    • Headers and global variables
    • Calling Java functions from Java and native threads
    • Implementation of other native functions
    • The JNI_OnLoad() function implementation
  • Building the native library
  • Using native functions in Java code
  • Unresolved issues and bugs

 

[*]Licence

This document and the code generated from it are subject to the following
licence:

[*]Introduction

While JNI is a pretty exciting way to greatly extend Android functionality
and port existing software to it, to date there is not a lot of detailed
documentation on how to create the native libraries and interface with them
from the Android’s Java Virtual Machine (JVM). This document aims at filling this
gap, by providing a comprehensive example of creating a native JNI library, and
using it from Java.

This document has been generated from source using
noweb, a literate programming tool. The
JNIExample.nw is the
source in noweb format. It can be used to generate the document output in a variety
of formats (for example,
PDF),
as well as generate the JNI example source code.

The complete Android project, including the source code generated from
JNIExample.nw is
available for
download. So,
if you are impatient, just grab it and check out the “Building the native library”
section [->], which describes prerequisites for the build and the build
procedure itself.

This document is not a replacement for other general JNI documentation. If you are
not familiar with JNI, you may want to have a look at the following resources:

Also, there are a couple of blog entries, which contain some bits of useful information:

If you notice any errors or omissions (there are a couple of known bugs and unresolved
issues [->]), or have a suggestion on how to improve this document, feel free
to contact me using the email address mentioned above.

 

[*]Java interface

We start by defining a Java class JNIExampleInterface, which will provide the
interface to calling the native functions, defined in a native (C++) library.
The native functions corresponding to Java functions will need to have matching
call signatures (i.e. the count and types of the arguments, as well as return
type). The easiest way to get the correct function signatures in the native
library is to first write down their Java prototypes, and then use the javah
tool to generate the native JNI header with native function prototypes. These
can be cut and pasted into the C++ file for implementation.

The Java functions which are backed by the corresponding native functions are
declared in a usual way, adding a native qualifier. We also want to
demonstrate how we could do the callbacks, i.e. calling the Java code from
native code. That leads to the following high-level view of our interface class:

 

One valid question about this definition is why we need a Handler class
attribute. It turns out that it will come in handy in situations, when the
native library wants to pass some information to the Java process through a
callback. If the callback will be called by a native thread (for extended
discussion see “Calling Java functions” section [->]), and then
will try to modify the
application’s user interface (UI) in any way, an exception will be thrown, as
Android only allows the thread which created the UI (the UI thread) to modify
it. To overcome this problem we are going to use the message-passing interface
provided by Handler to dispatch the data received by a callback to the UI
thread, and allow it to do the UI modifications. In order for this to work,
we are going to accept a Handler instance as an argument for non-trivial
constructor (reasons for keeping trivial one will become apparent later), and
save it in a class attribute, and that’s pretty much the only task for the
constructor:

 

To illustrate various argument-passing techniques, we define three native
functions:

  • callVoid(): takes no arguments and returns nothing;
  • getNewData(): takes two arguments and constructs a new class
    instance using them;
  • getDataString(): extracts a value from an object, which is
    passed as an argument.

 

The callback will receive a string as an argument, and dispatch it to the
Handler instance recorded in the constructor, after wrapping it in
a Bundle:

 

We also need a definition of a dummy Data class, used purely for
illustrative purposes:

 

After the source files Data.java and JNIExampleInterface.java are compiled,
we can generate the JNI header file, containing the prototypes
of the native functions, corresponding to their Java counterparts:

 

Native library implementation

At a high level, the Java library (consisting, in this case, of a single
source file JNIExample.cpp) will look like that:

 

 

Headers and global variables

The following includes define the functions provided by Android’s version
of JNI, as well as some useful helpers:

 

Various other things which will come in handy:

 

It is useful to have some global variables to cache things which we know will
not change during the lifetime of our program, and can be safely used across
multiple threads. One of such things is the JVM handle. We can retrieve it
every time it’s needed (for example, using android::AndroidRuntime::getJavaVM()
function), but as it does not change, it’s better to cache it.

We can also use global variables to cache the references
to required classes. As described below, it is not always easy to do class
resolution in native code, especially when it is done from native threads (see
“Calling Java functions” section [->] for details).
Here we are just providing the global variables to hold instances of Data
and JNIExampleInterface class objects, as well as defining some constant
strings which will come in handy:

 

Defines gJavaVM, jobject, kDataPath, kInterfacePath (links are to index).

 

[*]Calling Java functions from Java and native threads

The callVoid() function is the simplest one, as it does not take any
arguments, and returns nothing. We will use it to illustrate how the data
can be passed back to Java through the callback mechanism, by calling
the Java callBack() function.

At this point it is important to recognize that there are two distinct
possibilities here: the Java function may be called either from a thread which
originated in Java or from a native thread, which has been started in the native
code, and of which JVM has no knowledge of. In the former case the call may be
performed directly, in the latter we must first attach the native thread to the
JVM. That requires an additional layer, a native callback handler, which will
do the right thing in either case. We will also need a function to create the
native thread, so structurally the implementation will look like this:

 

Native callback handler gets the JNI environment (attaching the native
thread if necessary), uses a cached reference to the
gInterfaceObject to get to JNIExampleInterface class,
obtains callBack() method reference, and calls it:

 

Defines callback_handler (links are to index).

A few comments are in order:

  • The JNI environment, returned by the JNI GetEnv() function is unique
    for each thread, so must be retrieved every time we enter the function. The
    JavaVM pointer, on the other hand, is per-program, so can be cached (you
    will see it done in the JNI_OnLoad() function), and safely used across
    threads. 

     

  • When we attach a native thread, the associated Java environment comes
    with a bootstrap class loader. That means that even if we would try to get a
    class reference in the function (the normal way to do it would be to use
    FindClass() JNI function), it would trigger an exception. Because of that
    we use a cached copy of JNIExampleInterface object to get a class
    reference (amusingly, we cannot cache the reference to the class itself, as
    any attempt to use it triggers an exception from JVM, who thinks that such
    reference should not be visible to native code). This caching is also done
    in JNI_OnLoad(), which might be the only function called by Android Java
    implementation with a functional class loader. 

     

  • In order to retrieve the method ID of the callBack() method, we need
    to specify its name and JNI signature. In this case the signature
    indicates that the function takes a java.lang.String object as an argument,
    and returns nothing (i.e. has return type void). Consult JNI documentation
    for more information on function signatures, one useful tip is that you can use
    javap utility to look up the function signatures of non-native functions
    (for native functions the signature information is already included as comments
    into the header, generated by javah). 

     

  • Someone more paranoid than me could use locking to avoid race conditions
    associated with setting and checking of the isAttached variable.

In order to test calling from native threads, we will also need a function
which is started in a separate thread. Its only role is to call the callback
handler:

 

Defines native_thread_start (links are to index).

We now have all necessary pieces to implement the native counterpart of the
callVoid() function:

 

Defines JNICALL (links are to index).

 

Implementation of other native functions

The getNewData() function illustrates creation of a new Java object in
the native library, which is then returned to the caller. Again, we use a
cached Data object reference in order to obtain the class and create
a new instance.

 

Defines jobject (links are to index).

The getDataString() function illustrates how a value stored in an object’s
attribute can be retrieved in a native function.

 

Defines jstring (links are to index).

 

The JNI_OnLoad() function implementation

The JNI_OnLoad() function must be provided by the native library in order
for the JNI to work with Android JVM. It will be called immediately after the
native library is loaded into the JVM. We already mentioned a couple of tasks
which should be performed in this function: caching of the global JavaVM
pointer and caching of the object instances to enable us to call into Java.
In addition, any native methods which we want to call from Java must be
registered, otherwise Android JVM will not be able to resolve them. The
overall structure of the function thus can be written down as follows:

 

We need some way to cache a reference to a class, because native threads do not
have access to a functional classloader. As explained above, we can’t cache the
class references themselves, as it makes JVM unhappy. Instead we cache
instances of these classes, so that we can later retrieve class references
using GetObjectClass() JNI function. One thing to remember is that these
objects must be protected from garbage-collecting using NewGlobalRef(),
as that guarantees that they will remain available to different threads during
JVM lifetime. Creating the instances and storing them in the global variables
is the job for the <a href="#NWD1sCYeT-I">initClassHelper</a>() function:

 

Defines initClassHelper (links are to index).

With this function defined, class instance caching is trivial:

 

In order to register the native functions, we create an array
of JNINativeMethod structures, which contain
function names, signatures (they can be simply copied from the comments,
generated by javah), and pointers to the implementing functions. This
array is then passed to Android’s registerNativeMethods() function:

 

 

[*]Building the native library

In order to build the native library, you need to include
Android’s native headers and link against native libraries.
The only way I know to get those is to check out and build the entire
Android source code, and then build it. Procedure is described in detail at
Android Get Source
page. Make sure that you use the branch tag matching your SDK version,
for example code in the release-1.0 branch matches Android 1.1 SDK.

For an example of CXXFLAGS and LDFLAGS you need to use to
create a shared library with Android toolchain, check out the
Makefile, included in the
example project tarball.
They are derived from build/core/combo/linux-arm.mk in Android
source.

You will probably want to build the entire example project, so you
will need a copy of the SDK as well. This code has been tested to build
with Android’s 1.1 SDK and run on the currently released version of the
phone. Once you downloaded the SDK and the example tarball and
unpacked them, you can build the project using the command

 

Using native functions in Java code

We will now create a simple activity, taking advantage of the
JNI functions. One non-trivial task we will have to do in
onCreate() method of the activity is to load the native
JNI library, to make the functions defined there accessible to
Java. Overall structure:

 

Imports needed to draw the UI and display it to the user:

 

Imports needed to enable communication between the Java callback
and the UI thread:

 

Imports for manipulation with the native library:

 

We will also need access to our JNI interface class and toy Data
class:

 

Logging utilities will also come in handy:

 

At this time the only officialy supported way to create an Android application
is by using the Java API. That means, that no facilities are provided
to easily build and package shared libraries, and automatically load them
on application startup. One possible way to include the library into the
application package (file with extension .apk) is to place it into the
assets subdirectory of the Android project, created with activitycreator.
During the package build it will be automatically included into the APK package,
however we still will have to load it by hand when our application starts up.
Luckily, the location where APK is installed is known, and APK is simply a ZIP
archive, so we can extract the library file from Java and copy it into the
application directory, allowing us to load it:

 

The rest simply demonstrates the functionality, provided by the native
library, by calling the native functions and displaying the results.
For the callVoid() demo we need to initialize a handler first, and
pass it to the JNI interface class, to enable us to receive callback
messages:

 

We also set up a button which will call callVoid() from
the native library when pressed:

 

For getNewData() we pass the parameters to the native function
and expect to get the Data object back:

 

And pretty much the same for getDataString():

 

Try pushing the buttons and see whether it actually works!

 

[*]Unresolved issues and bugs

Even though the example is fully functional, there are a couple unresolved issues
remaining, which I was not able to figure out so far. Problems appear when you
start the activity, then press the Back button to hide it, and then start it
again. In my experience, calls to native functions in such restarted activity
will fail spectacularly. callVoid() simply crashes with a segmentation fault,
while calls to getNewData() and getDataString() cause JVM to abort
with an error, because it is no longer happy with the globally cached object
reference. It appears that activity restart somehow invalidates our cached
object references, even though they are protected with NewGlobalRef(), and the
activity is running within the original JVM (activity restart does not mean
that JVM itself is restarted). I don’t have a good explanation on why that
happens, so if you have any ideas, please let me know.


 

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