2. JNI 类型和数据结构

来自：JNI Types And Data Structures

1. Primitive Types

Primitive Types and Native Equivalents:

Java 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	not applicable

2. Reference Types

JNI reference hierarchy:

• jobject
  
  • jclass (java.lang.Class objects)
  • jstring (java.lang.String objects)
  • jthrowable (java.lang.Throwable objects)
  • jarray (arrays)
    jobjectArray (object arrays)
    jbooleanArray (boolean arrays)
    jbyteArray (byte arrays)
    jcharArray (char arrays)
    jshortArray (short arrays)
    jintArray (int arrays)
    jlongArray (long arrays)
    jfloatArray (float arrays)
    jdoubleArray (double arrays)

3. 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 */

4. The Value Type

The jvalue union type is used as the element type in argument arrays. 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;

5. Type Signatures

The JNI uses the Java VM’s representation of type signatures.

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

Eg:
Java method

long f (int n, String s, int[] arr);

The following type signatures:

//整体结构：(参数)返回值
//参数：I - int n；Ljava/lang/String - String s；[I - int[] arr
//返回值：J - long
(ILjava/lang/String;[I)J

6. Modified UTF-8 Strings

参考地址：Modified UTF-8 Strings