Interacting with Java from the Native Side |
The JNI uses thejarray
type to represent references to Java arrays. Similar tojstring
, you cannot directly accessjarray
types in your native method C code. Instead, you use functions provided by the JNI that allow you to obtain pointers to elements of integer arrays.Our second example,
IntArray.java
, contains a native method that totals up the contents of an integer array passed to it from a Java application. You cannot implement the native method by directly addressing the array elements. The following code snippet incorrectly tries to access the array elements directly:/* This program is illegal! */ JNIEXPORT jint JNICALL Java_IntArray_sumArray(JNIEnv *env, jobject obj, jintArray arr) { int i, sum = 0; for (i=0; i<10; i++) { sum += arr[i]; } ...The C program
IntArray.c
shows the correct way to implement the above functionJava_IntArray_sumArray
. In this example, you use one JNI function to get the length of the array. Use another JNI function to obtain a pointer to the individual elements of the array. Then, you can retrieve the elements. Lastly, use a third JNI function to release the array memory.Accessing Arrays of Primitive Elements
First, obtain the length of the array by calling the JNI function
GetArrayLength
.
Note: Unlike C language arrays, Java arrays carry length information.
JNIEXPORT jint JNICALL Java_IntArray_sumArray(JNIEnv *env, jobject obj, jintArray arr) { int i, sum = 0; jsize len = (*env)->GetArrayLength(env, arr); ...Next, obtain a pointer to the elements of the array. Our example contains an integer array so we use the JNI function
GetIntArrayElements
to obtain this pointer. Once you obtain the pointer, you can use normal C language operations on the resulting integer array.{ ... jint *body = (*env)->GetIntArrayElements(env, arr, 0); for (i=0; i<len; i++) { sum += body[i]; } ...The JNI provides a set of functions to obtain array element pointers; use the function that corresponds to the primitive type of the array. Had our example contained a float array
arr
, for example, we would have used the JNI functionGetFloatArrayElements
to obtain a pointer to its elements, as follows:In general, the garbage collector may move Java arrays. However, the Java Virtual Machine guarantees that the result of... int i; float sum = 0; ... jint *body = (*env)->GetFloatArrayElements(env, arr, 0); for (i=0; i<len; i++) { sum += body[i]; } ...GetIntArrayElements
points to a nonmovable array of integers. The JNI will either "pin" down the array or it will make a copy of the array into nonmovable memory. Because of this, the native code must callReleaseIntArrayElements
when it has finished using the array, as follows:ReleaseIntArrayElements enables the JNI to copy back and free the memory referenced by the... (*env)->ReleaseIntArrayElements(env, arr, body, 0); return sum; }body
parameter if it is a copy of the original Java array. The "copy back" action enables the calling program to obtain the new values of array elements that the native method may have modified.ReleaseIntArrayElements
will "unpin" the Java array if it has been pinned in memory.Similar to the
Get<type>ArrayElements
functions, the JNI provides a set ofRelease<type>ArrayElements
functions. Do not forget to call the appropriateRelease<type>ArrayElements
function, such asReleaseIntArrayElements
. If you forget to make this call, the array stays pinned for an extended period of time. Or, the Java Virtual Machine is unable to reclaim the memory used to store the nonmovable copy of the array.The JNI provides a set of functions to access arrays of every primitive type, including
boolean
,byte
,char
,short
,int
,long
,float
, anddouble
. Each function in the following table accesses elements in the specified Java type of array:JNI Functions for Accessing Arrays
Function Array Type GetBooleanArrayElements
boolean
GetByteArrayElements
byte
GetCharArrayElements
char
GetShortArrayElements
short
GetIntArrayElements
int
GetLongArrayElements
long
GetFloatArrayElements
float
GetDoubleArrayElements
double
The JNI also provides a set of functions to release arrays of different primitive types. The following table summarizes these functions.
JNI Functions for Releasing Arrays
Function Array Type ReleaseBooleanArrayElements
boolean
ReleaseByteArrayElements
byte
ReleaseCharArrayElements
char
ReleaseShortArrayElements
short
ReleaseIntArrayElements
int
ReleaseLongArrayElements
long
ReleaseFloatArrayElements
float
ReleaseDoubleArrayElements
double
Accessing a Small Number of Elements
Note that the
Get<type>ArrayElements
function might potentially copy the entire array. You may want to limit the number of elements that are copied, especially if your array is large. If you are only interested in a small number of elements in a (potentially) large array, you should instead use theGet/Set<type>ArrayRegion
functions. These functions allow you to access, via copying, a small set of elements in an array.Accessing Arrays of Objects
The JNI provides a separate set of functions to access elements of object arrays. You can use these functions to get and set individual object array elements.
Note: You cannot get all the object array elements at once.
GetObjectArrayElement
returns the object element at a given index.SetObjectArrayElement
updates the object element at a given index.
Interacting with Java from the Native Side |