Android JNI – String Operations

String Operations

NewString

jstring NewString(JNIEnv *env, const jchar *unicodeChars, jsize len);

Constructs a new java.lang.String object from an array of Unicode characters.

GetStringLength

jsize GetStringLength(JNIEnv *env, jstring string);

GetStringChars

const jchar * GetStringChars(JNIEnv *env, jstring string, jboolean *isCopy);

Returns a pointer to the array of Unicode characters of the string. This pointer is valid until ReleaseStringchars() is called.

If isCopy is not NULL, then *isCopy is set to JNI_TRUE if a copy is made; or it is set to JNI_FALSE if no copy is made.

ReleaseStringChars

void ReleaseStringChars(JNIEnv *env, jstring string, const jchar *chars);

Informs the VM that the native code no longer needs access to chars. The chars argument is a pointer obtained from string using GetStringChars().

NewStringUTF

jstring NewStringUTF(JNIEnv *env, const char *bytes);

Constructs a new java.lang.String object from an array of characters in modified UTF-8 encoding.

GetStringUTFLength

jsize GetStringUTFLength(JNIEnv *env, jstring string);

Returns the length in bytes of the modified UTF-8 representation of a string.

GetStringUTFChars

const char * GetStringUTFChars(JNIEnv *env, jstring string, jboolean *isCopy);

Returns a pointer to an array of bytes representing the string in modified UTF-8 encoding. This array is valid until it is released by ReleaseStringUTFChars().

If isCopy is not NULL, then *isCopy is set to JNI_TRUE if a copy is made; or it is set to JNI_FALSE if no copy is made.

ReleaseStringUTFChars

void ReleaseStringUTFChars(JNIEnv *env, jstring string, const char *utf);

Informs the VM that the native code no longer needs access to utf. The utf argument is a pointer derived from string using GetStringUTFChars().

GetStringRegion

void GetStringRegion(JNIEnv *env, jstring str, jsize start, jsize len, jchar *buf);

Copies len number of Unicode characters beginning at offset start to the given buffer buf.

Throws StringIndexOutOfBoundsException on index overflow.

GetStringUTFRegion

void GetStringUTFRegion(JNIEnv *env, jstring str, jsize start, jsize len, char *buf);

Translates len number of Unicode characters beginning at offset start into modified UTF-8 encoding and place the result in the given buffer buf.

Throws StringIndexOutOfBoundsException on index overflow.

GetStringCritical
ReleaseStringCritical

const jchar * GetStringCritical(JNIEnv *env, jstring string, jboolean *isCopy);
void ReleaseStringCritical(JNIEnv *env, jstring string, const jchar *carray);

The semantics of these two functions are similar to the existing Get/ReleaseStringChars functions. If possible, the VM returns a pointer to string elements; otherwise, a copy is made. However, there are significant restrictions on how these functions can be used. In a code segment enclosed by Get/ReleaseStringCritical calls, the native code must not issue arbitrary JNI calls, or cause the current thread to block.

The restrictions on Get/ReleaseStringCritical are similar to those on Get/ReleasePrimitiveArrayCritical.

Reference

https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/jniTOC.html 

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