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The Java_Prompt_getLine function calls the JNI function GetStringUTFto read the contents of the string The GetStringUTFChars function is available through the JNIEnv interface pointer It converts the jstring reference, typically represented by the Java virtual machine implementation as a Unicode sequence, into a C string represented in the UTF-8 format If you are certain that the original string contains only 7-bit ASCII characters, you may pass the converted string to regular C library functions such as printf (We will discuss how to handle non-ASCII strings in Section 82)
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JNIEXPORT jstring JNICALL Java_Prompt_getLine(JNIEnv *env, jobject obj, jstring prompt) { char buf[128]; const jbyte *str; str = (*env)->GetStringUTFChars(env, prompt, NULL); if (str == NULL) { return NULL; /* OutOfMemoryError already thrown */ } printf("%s", str); (*env)->ReleaseStringUTFChars(env, prompt, str); /* We assume here that the user does not type more than * 127 characters */ scanf("%s", buf); return (*env)->NewStringUTF(env, buf); }
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Do not forget to check the return value of GetStringUTFChars Because the Java virtual machine implementation needs to allocate memory to hold the UTF-8 string, there is a chance that memory allocation will fail When that happens, GetStringUTFChars returns NULL and throws an OutOfMemoryError exception As we will learn in 6, throwing an exception through the JNI is different from throwing an exception in the Java programming language A pending exception thrown through the JNI does not automatically change control ow in native C code Instead, we need to issue an explicit return statement in order to skip the remaining statements in the C function After Java_Prompt_getLine returns, the exception will be thrown in Promptmain, caller of the PromptgetLine native method 322 Freeing Native String Resources When your native code nishes using the UTF-8 string obtained through GetStringUTFChars, it calls ReleaseStringUTFChars Calling ReleaseStringUTFChars indicates that the native method no longer needs the UTF-8 string 25
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returned by GetStringUTFChars; thus the memory taken by the UTF-8 string can be freed Failure to call ReleaseStringUTFChars would result in a memory leak, which could ultimately lead to memory exhaustion 323 Constructing New Strings You can construct a new javalangString instance in the native method by calling the JNI function NewStringUTF The NewStringUTF function takes a C string with the UTF-8 format and constructs a javalangString instance The newly constructed javalangString instance represents the same sequence of Unicode characters as the given UTF-8 C string If the virtual machine cannot allocate the memory needed to construct the javalangString instance, NewStringUTF throws an OutOfMemoryError exception and returns NULL In this example, we do not need to check its return value because the native method returns immediately afterwards If NewStringUTF fails, the OutOfMemoryError exception will be thrown in the Promptmain method that issued the native method call If NewStringUTF succeeds, it returns a JNI reference to the newly constructed javalangString instance The new instance is returned by PromptgetLine and then assigned to the local variable input in Promptmain 324 Other JNI String Functions The JNI supports a number of other string-related functions, in addition to the GetStringUTFChars, ReleaseStringUTFChars, and NewStringUTF functions introduced earlier GetStringChars and ReleaseStringChars obtain string characters represented in the Unicode format These functions are useful when, for example, the operating system supports Unicode as the native string format UTF-8 strings are always terminated with the \0 character, whereas Unicode strings are not To nd out the number of Unicode characters in a jstring reference, JNI programmers can call GetStringLength To nd out how many bytes are needed to represent a jstring in the UTF-8 format, JNI programmers can either call the ANSI C function strlen on the result of GetStringUTFChars, or call the JNI function GetStringUTFLength on the jstring reference directly The third argument to GetStringChars and GetStringUTFChars requires additional explanation:
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const jchar * GetStringChars(JNIEnv *env, jstring str, jboolean *isCopy);
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