有没有一种简单的方法可以在 JNI 代码中将 Java 字符串转换为真正的 UTF-8 字节数组?
不幸的是,GetStringUTFChars() 几乎 完成了所需的但不完全是,它返回一个“修改过的”UTF-8 字节序列。主要区别在于修改后的 UTF-8 不包含任何空字符(因此您可以将其视为 ANSI C 空终止字符串),但另一个区别似乎是如何处理 Unicode 增补字符,例如表情符号。
像 U+1F604 "SMILING FACE WITH OPEN MOUTH AND SMILING EYES"这样的字符被存储为代理对(两个 UTF-16 字符 U+D83D U+DE04)并且有一个 4 字节的 UTF-8 等价物F0 9F 98 84,这是我在 Java 中将字符串转换为 UTF-8 时得到的字节序列:
char[] c = Character.toChars(0x1F604);
String s = new String(c);
System.out.println(s);
for (int i=0; i<c.length; ++i)
System.out.println("c["+i+"] = 0x"+Integer.toHexString(c[i]));
byte[] b = s.getBytes("UTF-8");
for (int i=0; i<b.length; ++i)
System.out.println("b["+i+"] = 0x"+Integer.toHexString(b[i] & 0xFF));
上面的代码打印如下:
😄 c[0] = 0xd83d c[1] = 0xde04 b[0] = 0xf0 b[1] = 0x9f b[2] = 0x98 b[3] = 0x84
但是,如果我将“s”传递给 native JNI 方法并调用 GetStringUTFChars(),我将获得 6 个字节。每个代理对字符都被独立地转换为 3 字节序列:
JNIEXPORT void JNICALL Java_EmojiTest_nativeTest(JNIEnv *env, jclass cls, jstring _s)
{
const char* sBytes = env->GetStringUTFChars(_s, NULL);
for (int i=0; sBytes[i]!=0; ++i)
fprintf(stderr, "%d: %02x\n", i, sBytes[i]);
env->ReleaseStringUTFChars(_s, sBytes);
return result;
}
0: ed 1: a0 2: bd 3: ed 4: b8 5: 84
Wikipedia UTF-8 article建议 GetStringUTFChars() 实际上返回 CESU-8 而不是 UTF-8。这反过来导致我的 native Mac 代码崩溃,因为它不是有效的 UTF-8 序列:
CFStringRef str = CFStringCreateWithCString(NULL, path, kCFStringEncodingUTF8);
CFURLRef url = CFURLCreateWithFileSystemPath(NULL, str, kCFURLPOSIXPathStyle, false);
我想我可以更改我所有的 JNI 方法以采用 byte[] 而不是 String 并在 Java 中进行 UTF-8 转换,但这看起来有点难看,是否有更好的解决方案?
最佳答案
这在 Java 文档中有清楚的解释:
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().
The JNI uses modified UTF-8 strings to represent various string types. Modified UTF-8 strings are the same as those used by the Java VM. Modified UTF-8 strings are encoded so that character sequences that contain only non-null ASCII characters can be represented using only one byte per character, but all Unicode characters can be represented.
All characters in the range
\u0001
to\u007F
are represented by a single byte, as follows:The seven bits of data in the byte give the value of the character represented.
The null character (
'\u0000'
) and characters in the range'\u0080'
to'\u07FF'
are represented by a pair of bytes x and y:The bytes represent the character with the value
((x & 0x1f) << 6) + (y & 0x3f)
.Characters in the range
'\u0800'
to'\uFFFF'
are represented by 3 bytes x, y, and z:The character with the value
((x & 0xf) << 12) + ((y & 0x3f) << 6) + (z & 0x3f)
is represented by the bytes.Characters with code points above U+FFFF (so-called supplementary characters) are represented by separately encoding the two surrogate code units of their UTF-16 representation. Each of the surrogate code units is represented by three bytes. This means, supplementary characters are represented by six bytes, u, v, w, x, y, and z:
The character with the value
0x10000+((v&0x0f)<<16)+((w&0x3f)<<10)+(y&0x0f)<<6)+(z&0x3f)
is represented by the six bytes.The bytes of multibyte characters are stored in the class file in big-endian (high byte first) order.
There are two differences between this format and the standard UTF-8 format. First, the null character (char)0 is encoded using the two-byte format rather than the one-byte format. This means that modified UTF-8 strings never have embedded nulls. Second, only the one-byte, two-byte, and three-byte formats of standard UTF-8 are used. The Java VM does not recognize the four-byte format of standard UTF-8; it uses its own two-times-three-byte format instead.
For more information regarding the standard UTF-8 format, see section 3.9 Unicode Encoding Forms of The Unicode Standard, Version 4.0.
由于U+1F604是增补字符,Java不支持UTF-8的4字节编码格式,所以U+1F604通过UTF-16代理对U+D83D U+DE04
的编码在修改后的UTF-8中表示。每个代理项使用 3 个字节,因此总共 6 个字节。
那么,回答你的问题...
Is there an easy way to convert a Java string to a true UTF-8 byte array in JNI code?
您可以:
使用
GetStringChars()
获取原始的 UTF-16 编码字符,然后从中创建您自己的 UTF-8 字节数组。从 UTF-16 到 UTF-8 的转换是一种非常简单的手动实现算法,或者您可以使用您的平台或第 3 方库提供的任何预先存在的实现。让您的 JNI 代码回调到 Java 中以调用
String.getBytes(String charsetName)
编码jstring
的方法对象到 UTF-8 字节数组,例如:JNIEXPORT void JNICALL Java_EmojiTest_nativeTest(JNIEnv *env, jclass cls, jstring _s) { const jclass stringClass = env->GetObjectClass(_s); const jmethodID getBytes = env->GetMethodID(stringClass, "getBytes", "(Ljava/lang/String;)[B"); const jstring charsetName = env->NewStringUTF("UTF-8"); const jbyteArray stringJbytes = (jbyteArray) env->CallObjectMethod(_s, getBytes, charsetName); env->DeleteLocalRef(charsetName); const jsize length = env->GetArrayLength(stringJbytes); const jbyte* pBytes = env->GetByteArrayElements(stringJbytes, NULL); for (int i = 0; i < length; ++i) fprintf(stderr, "%d: %02x\n", i, pBytes[i]); env->ReleaseByteArrayElements(stringJbytes, pBytes, JNI_ABORT); env->DeleteLocalRef(stringJbytes); }
The Wikipedia UTF-8 article suggests that GetStringUTFChars() actually returns CESU-8 rather than UTF-8
Java 的修改后的 UTF-8 与 CESU-8 不完全相同:
CESU-8 is similar to Java's Modified UTF-8 but does not have the special encoding of the NUL character (U+0000).
关于java - 在 Java JNI 中获取真正的 UTF-8 字符,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/32205446/