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am 8fc88562: am 85e21bd5: Mention ProGuard in the JNI documentation.

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* commit '8fc8856268218fbd286eeb0b2f34eec23cb74f8c':
  Mention ProGuard in the JNI documentation.
This commit is contained in:
Elliott Hughes
2014-11-27 18:49:58 +00:00
committed by Android Git Automerger
parent 022f4277c4 8fc8856268
commit 3e2848d249
1 changed files with 17 additions and 9 deletions
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26
docs/html/training/articles/perf-jni.jd
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@@ -635,20 +635,31 @@ avoid some problems.
<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>
@@ -656,12 +667,9 @@ class and uses that.</p>
<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>