Accessing Arrays of Objects in Java

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Accessing Arrays of Objects
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BASIC TYPES, STRINGS, AND ARRAYS
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The code that creates the second dimension is quite straightforward NewIntArray allocates the individual array elements, and SetIntArrayRegion copies the contents of the tmp[] buffer into the newly allocated one-dimensional arrays After completing the SetObjectArrayElement call, the jth element of the ith one-dimensional array has value i+j Running the ObjectArrayTestmain method produces the following output:
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0 1 2 1 2 3 2 3 4
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The DeleteLocalRef call at the end of the loop ensures that the virtual machine does not run out of the memory used to hold JNI references such as iarr Section 521 explains in detail when and why you need to call DeleteLocalRef
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C H A P T E R
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Fields and Methods
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NOW that you know how the JNI lets native code access primitive types and reference types such as strings and arrays, the next step will be to learn how to interact with elds and methods in arbitrary objects In addition to accessing elds, this includes making calls to methods implemented in the Java programming language from native code, commonly known as performing callbacks from native code We will begin by introducing the JNI functions that support eld access and method callbacks Later in this chapter we will discuss how to make such operations more ef cient by using a simple but effective caching technique In the last section, we will discuss the performance characteristics of calling native methods as well as accessing elds and calling methods from native code
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41 Accessing Fields
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The Java programming language supports two kinds of elds Each instance of a class has its own copy of the instance elds of the class, whereas all instances of a class share the static elds of the class The JNI provides functions that native code can use to get and set instance elds in objects and static elds in classes Let us rst look at an example program that illustrates how to access instance elds from a native method implementation
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Accessing Fields
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class InstanceFieldAccess { private String s; private native void accessField(); public static void main(String args[]) { InstanceFieldAccess c = new InstanceFieldAccess(); cs = "abc"; caccessField(); Systemoutprintln("In Java:"); Systemoutprintln(" cs = \"" + cs + "\""); } static { SystemloadLibrary("InstanceFieldAccess"); } }
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The InstanceFieldAccess class de nes an instance eld s The main method creates an object, sets the instance eld, and then calls the native method InstanceFieldAccessaccessField As we will see shortly, the native method prints out the existing value of the instance eld and then sets the eld to a new value The program prints the eld value again after the native method returns, demonstrating that the eld value has indeed changed Here is the implementation of the InstanceFieldAccessaccessField native method
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JNIEXPORT void JNICALL Java_InstanceFieldAccess_accessField(JNIEnv *env, jobject obj) { jfieldID fid; /* store the field ID */ jstring jstr; const char *str; /* Get a reference to obj s class */ jclass cls = (*env)->GetObjectClass(env, obj); printf("In C:\n"); /* Look for the instance field s in cls */ fid = (*env)->GetFieldID(env, cls, "s", "Ljava/lang/String;"); if (fid == NULL) { return; /* failed to find the field */ }
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Procedure for Accessing an Instance Field
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/* Read the instance field s */ jstr = (*env)->GetObjectField(env, obj, fid); str = (*env)->GetStringUTFChars(env, jstr, NULL); if (str == NULL) { return; /* out of memory */ } printf(" cs = \"%s\"\n", str); (*env)->ReleaseStringUTFChars(env, jstr, str); /* Create a new string and overwrite the instance field */ jstr = (*env)->NewStringUTF(env, "123"); if (jstr == NULL) { return; /* out of memory */ } (*env)->SetObjectField(env, obj, fid, jstr); }
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Running the InstanceFieldAccess class with the InstanceFieldAccess native library produces the following output:
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In C: cs = "abc" In Java: cs = "123"
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411 Procedure for Accessing an Instance Field To access an instance eld, the native method follows a two-step process First, it calls GetFieldID to obtain the eld ID from the class reference, eld name, and eld descriptor:
fid = (*env)->GetFieldID(env, cls, "s", "Ljava/lang/String;");
The example code obtains the class reference cls by calling GetObjectClass on the instance reference obj, which is passed as the second argument to the native method implementation Once you have obtained the eld ID, you can pass the object reference and the eld ID to the appropriate instance eld access function:
jstr = (*env)->GetObjectField(env, obj, fid);