Merge "Mention ProGuard in the JNI documentation."

<a name="faq_FindClass" id="faq_FindClass"></a>

<h2>FAQ: Why didn't <code>FindClass</code> find my class?</h2>

<p>(Most of this advice applies equally well to failures to find methods

with <code>GetMethodID</code> or <code>GetStaticMethodID</code>, or fields

with <code>GetFieldID</code> or <code>GetStaticFieldID</code>.)</p>

<p>Make sure that the class name string has the correct format.  JNI class

names start with the package name and are separated with slashes,

such as <code>java/lang/String</code>.  If you're looking up an array class,

you need to start with the appropriate number of square brackets and

must also wrap the class with 'L' and ';', so a one-dimensional array of

<code>String</code> would be <code>[Ljava/lang/String;</code>.</p>

<code>String</code> would be <code>[Ljava/lang/String;</code>.

If you're looking up an inner class, use '$' rather than '.'. In general,

using <code>javap</code> on the .class file is a good way to find out the

internal name of your class.</p>

<p>If you're using ProGuard, make sure that

<a href="{@docRoot}tools/help/proguard.html#configuring">ProGuard didn't

strip out your class</a>. This can happen if your class/method/field is only

used from JNI.

<p>If the class name looks right, you could be running into a class loader

issue.  <code>FindClass</code> wants to start the class search in the

class loader associated with your code.  It examines the call stack,

which will look something like:

<pre>    Foo.myfunc(Native Method)

    Foo.main(Foo.java:10)

    dalvik.system.NativeStart.main(Native Method)</pre>

    Foo.main(Foo.java:10)</pre>

<p>The topmost method is <code>Foo.myfunc</code>.  <code>FindClass</code>

finds the <code>ClassLoader</code> object associated with the <code>Foo</code>

<p>This usually does what you want.  You can get into trouble if you

create a thread yourself (perhaps by calling <code>pthread_create</code>

and then attaching it with <code>AttachCurrentThread</code>).

Now the stack trace looks like this:</p>

<pre>    dalvik.system.NativeStart.run(Native Method)</pre>

<p>The topmost method is <code>NativeStart.run</code>, which isn't part of

your application.  If you call <code>FindClass</code> from this thread, the

and then attaching it with <code>AttachCurrentThread</code>). Now there

are no stack frames from your application.

If you call <code>FindClass</code> from this thread, the

JavaVM will start in the "system" class loader instead of the one associated

with your application, so attempts to find app-specific classes will fail.</p>