From 4387335ba33344efcfa2105fa098cc877744003f Mon Sep 17 00:00:00 2001 From: Elliott Hughes Date: Wed, 20 Jul 2011 15:02:35 -0700 Subject: [PATCH] Manually merge the changes made in honeycomb-mr2 to get this on the web. We'd made good changes in both branches. Change-Id: I64fb4f1ff06da8767c5a6cee9fbcc2b955d6e207 --- docs/html/guide/practices/design/jni.jd | 243 +++++++++++------------- 1 file changed, 112 insertions(+), 131 deletions(-) diff --git a/docs/html/guide/practices/design/jni.jd b/docs/html/guide/practices/design/jni.jd index 39624f51386d0..ea318bf9b90c5 100644 --- a/docs/html/guide/practices/design/jni.jd +++ b/docs/html/guide/practices/design/jni.jd @@ -19,9 +19,9 @@ page.title=JNI Tips
  • Native Libraries
  • 64-bit Considerations
  • Unsupported Features
    • -
  • FAQ: UnsatisfiedLinkError
    • -
  • FAQ: FindClass didn't find my class
    • -
  • FAQ: Sharing raw data with native code
    • +
  • FAQ: Why do I get UnsatisfiedLinkError
    • +
  • FAQ: Why didn't FindClass find my class?
    • +
  • FAQ: How do I share raw data with native code?
    • @@ -32,7 +32,7 @@ page.title=JNI Tips

    JNI is the Java Native Interface. It defines a way for code written in the Java programming language to interact with native -code, e.g. functions written in C/C++. It's VM-neutral, has support for loading code from +code: functions written in C/C++. It's VM-neutral, has support for loading code from dynamic shared libraries, and while cumbersome at times is reasonably efficient.

    You really should read through the @@ -40,8 +40,7 @@ dynamic shared libraries, and while cumbersome at times is reasonably efficient. to get a sense for how JNI works and what features are available. Some aspects of the interface aren't immediately obvious on first reading, so you may find the next few sections handy. -The more detailed JNI Programmer's Guide and Specification can be found -here.

    +There's a more detailed JNI Programmer's Guide and Specification.

    @@ -59,20 +58,20 @@ the first argument.

    On some VMs, the JNIEnv is used for thread-local storage. For this reason, you cannot share a JNIEnv between threads. If a piece of code has no other way to get its JNIEnv, you should share -the JavaVM, and use JavaVM->GetEnv to discover the thread's JNIEnv. (Assuming it has one; see AttachCurrentThread below.)

    +the JavaVM, and use GetEnv to discover the thread's JNIEnv. (Assuming it has one; see AttachCurrentThread below.)

    The C declarations of JNIEnv and JavaVM are different from the C++ -declarations. "jni.h" provides different typedefs -depending on whether it's included into ".c" or ".cpp". For this reason it's a bad idea to +declarations. The "jni.h" include file provides different typedefs +depending on whether it's included into C or C++. For this reason it's a bad idea to include JNIEnv arguments in header files included by both languages. (Put another way: if your -header file requires "#ifdef __cplusplus", you may have to do some extra work if anything in +header file requires #ifdef __cplusplus, you may have to do some extra work if anything in that header refers to JNIEnv.)

    Threads

    All VM threads are Linux threads, scheduled by the kernel. They're usually -started using Java language features (notably Thread.start()), +started using Java language features (notably Thread.start), but they can also be created elsewhere and then attached to the VM. For example, a thread started with pthread_create can be attached with the JNI AttachCurrentThread or @@ -91,7 +90,7 @@ request, the VM will pause the thread the next time it makes a JNI call.

    Threads attached through JNI must call DetachCurrentThread before they exit. -If coding this directly is awkward, in Android >= 2.0 ("Eclair") you +If coding this directly is awkward, in Android 2.0 (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. (Use that @@ -108,7 +107,7 @@ the argument.)

    @@ -118,12 +117,12 @@ comparisons, but once you have them the actual call to get the field or invoke t is very quick.

    If performance is important, it's useful to look the values up once and cache the results -in your native code. Because we are limiting ourselves to one VM per process, it's reasonable +in your native code. Because there is a limit of one VM per process, it's reasonable to store this data in a static local structure.

    The class references, field IDs, and method IDs are guaranteed valid until the class is unloaded. Classes are only unloaded if all classes associated with a ClassLoader can be garbage collected, -which is rare but will not be impossible in our system. Note however that +which is rare but will not be impossible in Android. Note however that the jclass is a class reference and must be protected with a call to NewGlobalRef (see the next section).

    @@ -134,27 +133,18 @@ the IDs is to add a piece of code that looks like this to the appropriate class: 
        /*
      * We use a class initializer to allow the native code to cache some
-     * field offsets.
+     * field offsets. This native function looks up and caches interesting
+     * class/field/method IDs. Throws on failure.
      */
+    private static native void nativeInit();
 
-    /*
-     * A native function that looks up and caches interesting
-     * class/field/method IDs for this class.  Returns false on failure.
-     */
-    native private static boolean nativeClassInit();
-
-    /*
-     * Invoke the native initializer when the class is loaded.
-     */
     static {
-        if (!nativeClassInit())
-            throw new RuntimeException("native init failed");
+        nativeInit();
     }
    -

    Create a nativeClassInit method in your C/C++ code that performs the ID lookups. The code +

    Create a nativeClassInit method in your C/C++ code that performs the ID lookups. The code will be executed once, when the class is initialized. If the class is ever unloaded and -then reloaded, it will be executed again. (See the implementation of java.io.FileDescriptor -for an example in our source tree.)

    +then reloaded, it will be executed again.

    Local and Global References

    @@ -179,12 +169,12 @@ from FindClass, e.g.:

    jclass globalClass = reinterpret_cast<jclass>(env->NewGlobalRef(localClass));

    All JNI methods accept both local and global references as arguments. -It's possible for references to the same object to have different values; -for example, the return values from consecutive calls to +It's possible for references to the same object to have different values. +For example, the return values from consecutive calls to NewGlobalRef on the same object may be different. To see if two references refer to the same object, you must use the IsSameObject function. Never compare -references with "==" in native code.

    +references with == in native code.

    One consequence of this is that you must not assume object references are constant or unique @@ -201,13 +191,13 @@ VM is only required to reserve slots for 16 local references, so if you need more than that you should either delete as you go or use EnsureLocalCapacity to reserve more.

    -

    Note: method and field IDs are just 32-bit identifiers, not object +

    Note that jfieldIDs and jmethodIDs are just integers, not object references, and should not be passed to NewGlobalRef. The raw data pointers returned by functions like GetStringUTFChars and GetByteArrayElements are also not objects.

    One unusual case deserves separate mention. If you attach a native -thread to the VM with AttachCurrentThread, the code you are running will +thread to the VM with AttachCurrentThread, the code you are running will never "return" to the VM until the thread detaches from the VM. Any local references you create will have to be deleted manually unless you're going to detach the thread soon.

    @@ -221,17 +211,17 @@ The nice thing about this is that you can count on having C-style zero-terminate suitable for use with standard libc string functions. The down side is that you cannot pass arbitrary UTF-8 data into the VM and expect it to work correctly.

    -

    It's usually best to operate with UTF-16 strings. With our current VMs, the +

    It's usually best to operate with UTF-16 strings. With Android's current VMs, the GetStringChars method does not require a copy, whereas GetStringUTFChars requires a malloc and a UTF conversion. Note that UTF-16 strings are not zero-terminated, and \u0000 is allowed, so you need to hang on to the string length as well as the string pointer.

    -

    Don't forget to Release the strings you Get. The +

    Don't forget to Release the strings you Get. The string functions return jchar* or jbyte*, which are C-style pointers to primitive data rather than local references. They -are guaranteed valid until Release is called, which means they are not +are guaranteed valid until Release is called, which means they are not released when the native method returns.

    Data passed to NewStringUTF must be in Modified UTF-8 format. A @@ -258,8 +248,8 @@ allocate some memory and make a copy. Either way, the raw pointer returned is guaranteed to be valid until the corresponding Release call is issued (which implies that, if the data wasn't copied, the array object will be pinned down and can't be relocated as part of compacting the heap). -You must Release every array you Get. Also, if the Get -call fails, you must ensure that your code doesn't try to Release a NULL +You must Release every array you Get. Also, if the Get +call fails, you must ensure that your code doesn't try to Release a NULL pointer later.

    You can determine whether or not the data was copied by passing in a @@ -302,12 +292,12 @@ 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.

    -

    Some have asserted that you can skip the Release call if +

    It is a common mistake (repeated in example code) to assume that you can skip the Release call if *isCopy is false. 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.

    -

    Also note that the JNI_COMMIT flag does NOT release the array, +

    Also note that the JNI_COMMIT flag does not release the array, and you will need to call Release again with a different flag eventually.

    @@ -319,8 +309,7 @@ eventually.

    and GetStringChars that may be very helpful when all you want to do is copy data in or out. Consider the following:

    -
    -    jbyte* data = env->GetByteArrayElements(array, NULL);
+
        jbyte* data = env->GetByteArrayElements(array, NULL);
     if (data != NULL) {
         memcpy(buffer, data, len);
         env->ReleaseByteArrayElements(array, data, JNI_ABORT);
@@ -329,12 +318,11 @@ to do is copy data in or out.  Consider the following:
    
 
    This grabs the array, copies the first len byte
 elements out of it, and then releases the array.  Depending upon the VM
 policies the Get call will either pin or copy the array contents.
-We copy the data (for perhaps a second time), then call Release; in this case
-we use JNI_ABORT so there's no chance of a third copy.
    
+The code copies the data (for perhaps a second time), then calls Release; in this case
+JNI_ABORT ensures there's no chance of a third copy.
    
 
-
    We can accomplish the same thing with this:
    
-
    -    env->GetByteArrayRegion(array, 0, len, buffer);
    
+
    One can accomplish the same thing more simply:
    
+
        env->GetByteArrayRegion(array, 0, len, buffer);
    
 
 
    This has several advantages:
    
 
      
@@ -353,29 +341,29 @@ to copy data into an array, and GetStringRegion or
 
 
      Exception
      
 
-
      You may not call most JNI functions while an exception is pending.
+
      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,
+ExceptionCheck, or ExceptionOccurred) and return,
 or clear the exception and handle it.
      
 
 
      The only JNI functions that you are allowed to call while an exception is
 pending are:
      
 
        
-    
      • DeleteGlobalRef
-    
      • DeleteLocalRef
-    
      • DeleteWeakGlobalRef
-    
      • ExceptionCheck
-    
      • ExceptionClear
-    
      • ExceptionDescribe
-    
      • ExceptionOccurred
-    
      • MonitorExit
-    
      • PopLocalFrame
-    
      • PushLocalFrame
-    
      • Release<PrimitiveType>ArrayElements
-    
      • ReleasePrimitiveArrayCritical
-    
      • ReleaseStringChars
-    
      • ReleaseStringCritical
-    
      • ReleaseStringUTFChars
+    
      • DeleteGlobalRef
+    
      • DeleteLocalRef
+    
      • DeleteWeakGlobalRef
+    
      • ExceptionCheck
+    
      • ExceptionClear
+    
      • ExceptionDescribe
+    
      • ExceptionOccurred
+    
      • MonitorExit
+    
      • PopLocalFrame
+    
      • PushLocalFrame
+    
      • Release<PrimitiveType>ArrayElements
+    
      • ReleasePrimitiveArrayCritical
+    
      • ReleaseStringChars
+    
      • ReleaseStringCritical
+    
      • ReleaseStringUTFChars
 
      
 
 
      Many JNI calls can throw an exception, but often provide a simpler way
@@ -396,86 +384,80 @@ native code, the exception will be noted and handled appropriately.
      
 ExceptionClear.  As usual,
 discarding exceptions without handling them can lead to problems.
      
 
-
      There are no built-in functions for manipulating the Throwable object
+
      There are no built-in functions for manipulating the Throwable object
 itself, so if you want to (say) get the exception string you will need to
-find the Throwable class, look up the method ID for
+find the Throwable class, look up the method ID for
 getMessage "()Ljava/lang/String;", invoke it, and if the result
 is non-NULL use GetStringUTFChars to get something you can
-hand to printf or a LOG macro.
      
+hand to printf(3) or equivalent.
      
 
 
 
 
      Extended Checking
      
 
-
      JNI does very little error checking.  Calling SetIntField
-on an Object field will succeed, even if the field is marked
-private and final.  The
-goal is to minimize the overhead on the assumption that, if you've written it in native code,
-you probably did it for performance reasons.
      
+
      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.
      
 
-
      In Dalvik, you can enable additional checks by setting the
-"-Xcheck:jni" flag.  If the flag is set, the VM directs
-the JavaVM and JNIEnv pointers to a different table of functions.
-These functions perform an extended series of checks before calling the
-standard implementation.
      
-
-
      The additional tests include:
      
+
      The additional checks include:
      
 
 
        
-
      •  Check for null pointers where not allowed.
        • 
-
      •  Verify argument type correctness (jclass is a class object,
-jfieldID points to field data, jstring is a java.lang.String).
        • 
-
      •  Field type correctness, e.g. don't store a HashMap in a String field.
        • 
-
      •  Ensure jmethodID is appropriate when making a static or virtual
-method call.
        • 
-
      •  Check to see if an exception is pending on calls where pending exceptions are not legal.
        • 
-
      •  Check for calls to inappropriate functions between Critical get/release calls.
        • 
-
      •  Check that JNIEnv structs aren't being shared between threads.
        • 
-
      •  Make sure local references aren't used outside their allowed lifespan.
        • 
-
      •  UTF-8 strings contain only valid Modified UTF-8 data.
        • 
+
      • Arrays: attempting to allocate a negative-sized array.
        • 
+
      • Bad pointers: passing a bad jarray/jclass/jobject/jstring to a JNI call, or passing a NULL pointer to a JNI call with a non-nullable argument.
        • 
+
      • Class names: passing anything but the “java/lang/String” style of class name to a JNI call.
        • 
+
      • Critical calls: making a JNI call between a “critical” get and its corresponding release.
        • 
+
      • Direct ByteBuffers: passing bad arguments to NewDirectByteBuffer.
        • 
+
      • Exceptions: making a JNI call while there’s an exception pending.
        • 
+
      • JNIEnv*s: using a JNIEnv* from the wrong thread.
        • 
+
      • jfieldIDs: using a NULL jfieldID, or using a jfieldID to set a field to a value of the wrong type (trying to assign a StringBuilder to a String field, say), or using a jfieldID for a static field to set an instance field or vice versa, or using a jfieldID from one class with instances of another class.
        • 
+
      • jmethodIDs: using the wrong kind of jmethodID when making a Call*Method JNI call: incorrect return type, static/non-static mismatch, wrong type for ‘this’ (for non-static calls) or wrong class (for static calls).
        • 
+
      • References: using DeleteGlobalRef/DeleteLocalRef on the wrong kind of reference.
        • 
+
      • Release modes: passing a bad release mode to a release call (something other than 0, JNI_ABORT, or JNI_COMMIT).
        • 
+
      • Type safety: returning an incompatible type from your native method (returning a StringBuilder from a method declared to return a String, say).
        • 
+
      • UTF-8: passing an invalid Modified UTF-8 byte sequence to a JNI call.
        • 
 
      
 
-
      Accessibility of methods and fields (i.e. public vs. private) is not
-checked.
      
+
      (Accessibility of methods and fields is still not checked: access restrictions don't apply to native code.)
      
 
-
      For a description of how to enable CheckJNI for Android apps, see
-Controlling the Embedded VM.
-It's currently enabled by default in the Android emulator and on
-"engineering" device builds.
      
+
      There are several ways to enable CheckJNI.
      
+
+
      If you’re using the emulator, CheckJNI is on by default.
      
+
+
      If you have a rooted device, you can use the following sequence of commands to restart the runtime with CheckJNI enabled:
      
+
+
      adb shell stop
+adb shell setprop dalvik.vm.checkjni true
+adb shell start
      
+
+
      In either of these cases, you’ll see something like this in your logcat output when the runtime starts:
      
+
+
      D AndroidRuntime: CheckJNI is ON
      
+
+
      If you have a regular device, you can use the following command:
      
+
+
      adb shell setprop debug.checkjni 1
      
+
+
      This won’t affect already-running apps, but any app launched from that point on will have CheckJNI enabled. (Change the property to any other value or simply rebooting will disable CheckJNI again.) In this case, you’ll see something like this in your logcat output the next time an app starts:
      
+
+
      D Late-enabling CheckJNI
      
 
-
      JNI checks can be modified with the -Xjniopts command-line
-flag.  Currently supported values include:
      
 
-
      
-
      forcecopy
-
      When set, any function that can return a copy of the original data
-(array of primitive values, UTF-16 chars) will always do so.  The buffers
-are over-allocated and surrounded with a guard pattern to help identify
-code writing outside the buffer, and the contents are erased before the
-storage is freed to trip up code that uses the data after calling Release.
-This will have a noticeable performance impact on some applications.
-
      warnonly
-
      By default, JNI "warnings" cause the VM to abort.  With this flag
-it continues on.
-
      
 
 
 
 
      Native Libraries
      
 
 
      You can load native code from shared libraries with the standard
-System.loadLibrary() call.  The
+System.loadLibrary call.  The
 preferred way to get at your native code is:
      
 
 
        
-
      •  Call System.loadLibrary() from a static class
+
      •  Call System.loadLibrary from a static class
 initializer.  (See the earlier example, where one is used to call
-nativeClassInit().)  The argument is the "undecorated"
-library name, e.g. to load "libfubar.so" you would pass in "fubar".
        • 
+nativeClassInit.)  The argument is the "undecorated"
+library name, so to load "libfubar.so" you would pass in "fubar".
 
      •  Provide a native function: jint JNI_OnLoad(JavaVM* vm, void* reserved)
        • 
 
      • In JNI_OnLoad, register all of your native methods.  You
 should declare
-the functions static so the names don't take up space in the symbol table
+the methods "static" so the names don't take up space in the symbol table
 on the device.
        • 
 
      
 
@@ -494,7 +476,7 @@ written in C++:
      
     return JNI_VERSION_1_6;
 }
    
 
-
    You can also call System.load() with the full path name of the
+
    You can also call System.load with the full path name of the
 shared library.  For Android apps, you may find it useful to get the full
 path to the application's private data storage area from the context object.
    
 
@@ -553,28 +535,28 @@ that use 64-bit pointers, you need to stash your native pointers in a
 
 
    For backward compatibility, you may need to be aware of:
    
 
      
-    
    • Until Android 2.0 ("Eclair"), the '$' character was not properly
+    
    • Until Android 2.0 (Eclair), the '$' character was not properly
     converted to "_00024" during searches for method names.  Working
     around this requires using explicit registration or moving the
     native methods out of inner classes.
-    
    • Until Android 2.0 ("Eclair"), it was not possible to use a pthread_key_create
+    
    • Until Android 2.0 (Eclair), it was not possible to use a pthread_key_create
     destructor function to avoid the VM's "thread must be detached before
     exit" check.  (The VM also uses a pthread key destructor function,
     so it'd be a race to see which gets called first.)
-    
    • "Weak global" references were not implemented until Android 2.2 ("Froyo").
+    
    • Until Android 2.2 (Froyo), weak global references were not implemented.
     Older VMs will vigorously reject attempts to use them.  You can use
     the Android platform version constants to test for support.
 
    
 
 
 
-
    FAQ: UnsatisfiedLinkError
    
+
    FAQ: Why do I get UnsatisfiedLinkError?
    
 
 
    When working on native code it's not uncommon to see a failure like this:
    
 
    java.lang.UnsatisfiedLinkError: Library foo not found
    
 
 
    In some cases it means what it says — the library wasn't found.  In
-other cases the library exists but couldn't be opened by dlopen(), and
+other cases the library exists but couldn't be opened by dlopen(3), and
 the details of the failure can be found in the exception's detail message.
    
 
 
    Common reasons why you might encounter "library not found" exceptions:
    
@@ -605,7 +587,7 @@ Some common reasons for this are:
    
         
  • For lazy method lookup, failing to declare C++ functions
         with extern "C".  You can use arm-eabi-nm
         to see the symbols as they appear in the library; if they look
-        mangled (e.g. _Z15Java_Foo_myfuncP7_JNIEnvP7_jclass
+        mangled (something like _Z15Java_Foo_myfuncP7_JNIEnvP7_jclass
         rather than Java_Foo_myfunc) then you need to
         adjust the declaration.
         
  • For explicit registration, minor errors when entering the
@@ -614,8 +596,7 @@ Some common reasons for this are:
    
         Remember that 'B' is byte and 'Z' is boolean.
         Class name components in signatures start with 'L', end with ';',
         use '/' to separate package/class names, and use '$' to separate
-        inner-class names
-        (e.g. Ljava/util/Map$Entry;).
+        inner-class names (Ljava/util/Map$Entry;, say).
     
 
 
@@ -624,11 +605,11 @@ avoid some problems.
 
 
 
-
    FAQ: FindClass didn't find my class
    
+
    FAQ: Why didn't FindClass find my class?
    
 
 
    Make sure that the class name string has the correct format.  JNI class
 names start with the package name and are separated with slashes,
-e.g. java/lang/String.  If you're looking up an array class,
+such as java/lang/String.  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
 String would be [Ljava/lang/String;.
    
@@ -667,8 +648,8 @@ with your application, so attempts to find app-specific classes will fail.
    
     If your app code is loading the library, FindClass
     will use the correct class loader.
     
  • Pass an instance of the class into the functions that need
-    it, e.g. declare your native method to take a Class argument and
-    then pass Foo.class in.
+    it, by declaring your native method to take a Class argument and
+    then passing Foo.class in.
     
  • Cache a reference to the ClassLoader object somewhere
     handy, and issue loadClass calls directly.  This requires
     some effort.
@@ -676,7 +657,7 @@ with your application, so attempts to find app-specific classes will fail.
    
 
 
 
-
    FAQ: Sharing raw data with native code
    
+
    FAQ: How do I share raw data with native code?
    
 
 
    You may find yourself in a situation where you need to access a large
 buffer of raw data from code written in Java and C/C++.  Common examples