[Android Pro] 小心ReleaseByteArrayElements 中的参数问题

referen to : http://blog.csdn.net/rainlight/article/details/818964

在Sun的官方文档中,关于该函数的用法如下

The array is returned to the calling Java language method, which in turn, garbage collects the reference to the array when it is no longer used. The array can be explicitly freed with the following call.

  (*env)-> ReleaseByteArrayElements(env, jb,
                                        (jbyte *)m, 0);

The last argument to the ReleaseByteArrayElements function above can have the following values:

• 0: Updates to the array from within the C code are reflected in the Java language copy.
• JNI_COMMIT: The Java language copy is updated, but the local jbyteArray is not freed.
• JNI_ABORT: Changes are not copied back, but the jbyteArray is freed. The value is used only if the array is obtained with a get mode of JNI_TRUE meaning the array is a copy.

小心最后一个参数,如果为0是会释放 m 所指向的内存的. 如果M刚好指向一个栈上的数组的话,这样可能在Release 版本中造成内存方面的随机错误.可以用JNI_COMMIT来避免.

其实现代码也许如下

+void
+KaffeJNI_ReleaseByteArrayElements(JNIEnv* env UNUSED, jbyteArray arr, jbyte* elems, jint mode)
+{
+ BEGIN_EXCEPTION_HANDLING_VOID();
+
+ if (elems != unhand_array((HArrayOfByte*)arr)->body) {
+ switch (mode) {
+ case JNI_COMMIT:
+ memcpy(unhand_array((HArrayOfByte*)arr)->body, elems, obj_length((HArrayOfByte*)arr) * sizeof(jbyte));
+ break;
+ case 0:
+ memcpy(unhand_array((HArrayOfByte*)arr)->body, elems, obj_length((HArrayOfByte*)arr) * sizeof(jbyte));
+ KFREE(elems);
+ break;
+ case JNI_ABORT:
+ KFREE(elems);
+ break;
+ }
+ }
+ END_EXCEPTION_HANDLING();
+}

JNI_COMMIT forces the native array to be copied back to the original array in the Java virtual machine. JNI_ABORT frees the memory allocated for the native array without copying back the new contents