JNI Function in Java

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JNI Function
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GetStringChars ReleaseStringChars
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GetStringUTFChars ReleaseStringUTFChars
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GetStringRegion SetStringRegion
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BASIC TYPES, STRINGS, AND ARRAYS
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Choosing among the String Functions
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327 Choosing among the String Functions Figure 32 illustrates how a programmer may choose among the string-related functions in JDK release 11 and Java 2 SDK release 12:
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12 and beyond Targeting release 11 or 12 Preallocated C string buffer, small xed-size strings, or small substrings N Any blocking or JNI calls while accessing string contents Y
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GetStringChars ReleaseStringChars GetStringUTFChars ReleaseStringUTFChars
GetStringRegion SetStringRegion GetStringUTFRegion SetStringUTFRegion
11 or both
GetStringCritical ReleaseStringCritical
Choosing among the JNI String Functions
If you are targeting 11 or both 11 and 12 releases, there is no choice other than Get/ReleaseStringChars and Get/ReleaseStringUTFChars If you are programming in Java 2 SDK release 12 and above, and you want to copy the contents of a string into an already-allocated C buffer, use GetStringRegion or GetStringUTFRegion For small xed-size strings, Get/SetStringRegion and Get/SetStringUTFRegion are almost always the preferred functions because the C buffer can be allocated on the C stack very cheaply The overhead of copying a small number of characters in the string is negligible One advantage of Get/SetStringRegion and Get/SetStringUTFRegion is that they do not perform memory allocation, and therefore never raise unexpected
Choosing among the String Functions
BASIC TYPES, STRINGS, AND ARRAYS
out-of-memory exceptions No exception checking is necessary if you make sure that index over ow cannot occur Another advantage of Get/SetStringRegion and Get/SetStringUTFRegion is that the you can specify a starting index and the number of characters These functions are suitable if the native code only needs to access a subset of characters in a long string GetStringCritical must be used with extreme care ( 325) You must make sure that while holding a pointer obtained through GetStringCritical, the native code does not allocate new objects in the Java virtual machine or perform other blocking calls that may cause the system to deadlock Here is an example that demonstrates the subtle issues in the use of GetStringCritical The following code obtains the content of a string and calls the fprintf function to write out the characters to the le handle fd:
/* This is not safe! */ const char *c_str = (*env)->GetStringCritical(env, j_str, 0); if (c_str == NULL) { /* error handling */ } fprintf(fd, "%s\n", c_str); (*env)->ReleaseStringCritical(env, j_str, c_str);
The problem with the above code is that it is not always safe to write to a le handle when garbage collection is disabled by the current thread Suppose, for example, that another thread T is waiting to read from the fd le handle Let us further assume that the operating system buffering is set up in such a way that the fprintf call waits until the thread T nishes reading all pending data from fd We have constructed a possible scenario for deadlocks: If thread T cannot allocate enough memory to serve as a buffer for reading from the le handle, it must request a garbage collection The garbage collection request will be blocked until the current thread executes ReleaseStringCritical, which cannot happen until the fprintf call returns The fprintf call is waiting, however, for thread T to nish reading from the le handle The following code, although similar to the example above, is almost certainly deadlock free:
/* This code segment is OK */ const char *c_str = (*env)->GetStringCritical(env, j_str, 0); if (c_str == NULL) { /* error handling */ } DrawString(c_str); (*env)->ReleaseStringCritical(env, j_str, c_str);
BASIC TYPES, STRINGS, AND ARRAYS
Accessing Arrays
DrawString is a system call that directly writes the string onto the screen Unless the screen display driver is also a Java application running in the same virtual machine, the DrawString function will not block inde nitely waiting for garbage collection to happen In summary, you need to consider all possible blocking behavior between a pair of Get/ReleaseStringCritical calls
33 Accessing Arrays
The JNI treats primitive arrays and object arrays differently Primitive arrays contain elements that are of primitive types such as int and boolean Object arrays contain elements that are of reference types such as class instances and other arrays For example, in the following code segment written in the Java programming language:
int[] iarr; float[] farr; Object[] oarr; int[][] arr2;
and farr are primitive arrays, whereas oarr and arr2 are object arrays Accessing primitive arrays in a native method requires the use of JNI functions similar to those used for accessing strings Let us look at a simple example The following program calls a native method sumArray that adds up the contents of an int array
iarr class IntArray { private native int sumArray(int[] arr); public static void main(String[] args) { IntArray p = new IntArray(); int arr[] = new int[10]; for (int i = 0; i < 10; i++) { arr[i] = i; } int sum = psumArray(arr); Systemoutprintln("sum = " + sum); } static { SystemloadLibrary("IntArray"); } }