Java Native Interface Specification - Chapter 2 : JNI Types and Data Structures
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JNI官方文档:https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/jniTOC.html
Chapter 3
This chapter discusses how the JNI maps Java types to native C types.
Primitive Types
Table?3-1 describes Java primitive types and their machine-dependent native equivalents.
Table?3-1 Primitive Types and Native EquivalentsJava Type | Native Type | Description |
---|---|---|
boolean | jboolean | unsigned 8 bits |
byte | jbyte | signed 8 bits |
char | jchar | unsigned 16 bits |
short | jshort | signed 16 bits |
int | jint | signed 32 bits |
long | jlong | signed 64 bits |
float | jfloat | 32 bits |
double | jdouble | 64 bits |
void | void | N/A |
The following definition is provided for convenience.
#define JNI_FALSE 0 #define JNI_TRUE 1
The jsize
integer type is used to describe cardinal indices and sizes:
typedef jint jsize;
Reference Types
The JNI includes a number of reference types that correspond to different kinds of Java objects. JNI reference types are organized in the hierarchy shown in Figure?3-1.
Figure?3-1 Reference Type Hierarchy
In C, all other JNI reference types are defined to be the same as jobject. For example:
typedef jobject jclass;
In C++, JNI introduces a set of dummy classes to enforce the subtyping relationship. For example:
class _jobject {}; class _jclass : public _jobject {}; ... typedef _jobject *jobject; typedef _jclass *jclass;
Field and Method IDs
Method and field IDs are regular C pointer types:
struct _jfieldID; /* opaque structure */ typedef struct _jfieldID *jfieldID; /* field IDs */ struct _jmethodID; /* opaque structure */ typedef struct _jmethodID *jmethodID; /* method IDs */
The Value Type
The jvalue
union type is used as the element type in argument arrays. It is declared as follows:
typedef union jvalue { jboolean z; jbyte b; jchar c; jshort s; jint i; jlong j; jfloat f; jdouble d; jobject l; } jvalue;
Type Signatures
The JNI uses the Java VM’s representation of type signatures. Table?3-2 shows these type signatures.
Table?3-2 Java VM Type Signatures
Type Signature | Java Type |
---|---|
Z | boolean |
B | byte |
C | char |
S | short |
I | int |
J | long |
F | float |
D | double |
L fully-qualified-class ; | fully-qualified-class |
[ type | type[] |
( arg-types ) ret-type | method type |
For example, the Java method:
long f (int n, String s, int[] arr);
has the following type signature:
(ILjava/lang/String;[I)J
Modified UTF-8 Strings
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:
0 | bits 6-0 |
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:
x:
1 | 1 | 0 | bits 10-6 |
y:
1 | 0 | bits 5-0 |
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:
x:
1 | 1 | 1 | 0 | bits 15-12 |
y:
1 | 0 | bits 11-6 |
z:
1 | 0 | bits 5-0 |
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:
u:
1 | 1 | 1 | 0 | 1 | 1 | 0 | 1 |
v:
1 | 0 | 1 | 0 | (bits 20-16) - 1 |
w:
1 | 0 | bits 15-10 |
x:
1 | 1 | 1 | 0 | 1 | 1 | 0 | 1 |
y:
1 | 0 | 1 | 1 | bits 9-6 |
z:
1 | 0 | bits 5-0 |
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.
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