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Überblick (JNA API)

This document is the API specification for the

library for simplified native library access for Java.

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JNA provides simplified access to native library methods without requiring any additional JNI or native code.

• Loading JNA
• Library Mapping
• Function Mapping
• Type Mapping
  • Primitive Types
  • Pointers
  • Strings
  • Wide (UNICODE) Strings
  • Primitive Arrays
  • Buffers/Memory Blocks
  • Callbacks/Function Pointers
  • Variable Argument Lists (Varargs)
  • Structures
  • Unions
  • Java Objects
  • Last Error
• Invocation Mapping
• Library Global Data
• VM Crash Protection
• Performance

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JNA includes a small, platform-specific shared library which enables all native access. When the

class is first accessed, JNA will first attempt to load this library from the directories specified in

. If that fails and

is set, it will fall back to loading from the system library paths. Finally it will attempt to extract the stub library from from the JNA jar file, and load it.

The

property is mainly to support jna.jar being included in -Xbootclasspath, where

and LD_LIBRARY_PATH are ignored. It is also useful for designating a version of the library to use in preference to any which may already be installed on the system.

Loading from the system may be enabled by

, and unpacking from the jar file may be disabled by

.

The library name used to search for JNA's native library may be altered by setting

, which defaults to "jnidispatch". It may be useful to set this value if your system requires unique names for shared libraries (rather than unique paths), or if your system must store different versions of the JNA shared library (e.g. for different architectures) in the same directory.

When you've determined which shared library holds the methods to which you need access, create a class corresponding to that library. For example, a mapping for the C library itself would look like one of the following:

// Alternative 1: interface-mapped class, dynamically load the C library
public interface CLibrary extends Library {
    CLibrary INSTANCE = (CLibrary)Native.load("c", CLibrary.class);
}

// Alternative 2: direct-mapped class (uses a concrete class rather than an
// interface, with a slight variation in method
// declarations). 
public class CLibrary {
    static {
        Native.register("c");
    }
}

The

passed to the

(or

) method is the undecorated name of the shared library file. Here are some examples of library name mappings.

Any given native library with a unique filesystem path is represented by a single instance of NativeLibrary and obtained via NativeLibrary.getInstance(String). The native library will be unloaded when no longer referenced by any Java code.

If the library name is null, your mappings will apply to the current process instead of a separately loaded library. This may help avoid conflicts if there are several incompatible versions of a library available.

The search path for loaded native libraries may be modified by setting jna.library.path and a few other properties. You may also bundle native libraries in a jar file and have JNA automatically extract them for loading. See NativeLibrary for details.

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Function names are mapped directly from their Java interface name to the symbol exported by the native library. For instance, the function to convert an ASCII string into an integer would look like this:

public interface CLibrary extends Library {
    int atol(String s);
}

Alternatively, you can map directly to a declared native method (with

):

public class CLibrary {
    public static native int atol(String s);
}

If you prefer to rename the Java methods to conform to Java coding conventions, then you can provide an entry (

/

) in the options

passed to

which maps the Java names to the native names. While this keeps your Java code a little cleaner, the additional mapping of names may make it a little less obvious the native functions being called.

An instance of the Function class is obtained through the NativeLibrary instance corresponding to the containing native library. This Function instance handles argument marshalling and delegation to the native function.

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Java types must be chosen to match native types of the same size. Following are the types supported by the JNA library.

NOTES

• Unsigned values may be passed by assigning the corresponding two's-complement representation to the signed type of the same size.
• Java arrays of primitive type may be wrapped by Buffer in order to access a subset of the array (changing the effective size and/or offest).
• Java arrays of primitive type and non-direct Buffers are only valid for use within the scope of a single call. If the native code keeps a reference to the memory, use Memory or direct ByteBuffers instead.
• Primitive arrays and structures as members of a structure are overlaid on the parent structure memory.
• Bitfields must be manually packed into an integer type.
• All other types must eventually be converted to one of the types in the this table. Methods with arguments or return values of types other than these must either use types deriving from NativeMapped or supply type conversion information for the unsupported types.
• Type mapping behavior may be customized by providing a TypeMapper for the Library.OPTION_TYPE_MAPPER option when initializing a library interface. See W32APITypeMapper for an example which provides custom conversion of boolean and String types. You are free to use whatever types are convenient in your defined interfaces, but all custom types must provide a mapping to one of the basic or derived types listed above.
• Type mapping may also be customized on a per-class basis for user-defined types by making the user-defined type implement the NativeMapped interface.
• Structure and Union are not converted to Pointer when passed by value.

Java primitive arrays may be used wherever a native primitive array is used. Any changes made by the native code to an array during a function call will be reflected in the Java array. If the native code will use the array outside of the function call where the array is provided,

or

should be used instead (see

).

To map a native multi-dimensional array, use a single-dimensional Java array with a number of elements equivalent to the full native array, e.g.

// Original C code
#define DIM0 2
#define DIM1 3
int array[DIM0][DIM1];
int i,j;

for (i=0;i < DIM0;i++) {
  for (j=0;j < DIM1;j++) {
    array[i][j] = i*DIM1 + j;
  }
}

// Equivalent JNA code
final int DIM0 = 2;
final int DIM1 = 3;
int[] array = new int[6];
for (int i=0;i < DIM0;i++) {
  for (int j=0;j < DIM1;j++) {
    array[i*DIM1 + j] = i*DIM1 + j;                   
  }                   
}

Pointers may be used as an opaque type from which other data types may be extracted. The Pointer type is a reasonable fallback for any pointer-based type (including arrays). The user is generally not allowed to construct a Pointer de novo.

Type-safe pointers may be defined by deriving from the PointerType class. Any such user-defined type will be treated the same as a Pointer.

Java

s perform the same function as the native types

and

(

-terminated arrays). In order to use the proper type when calling a native function, we have to introduce some sort of annotation to identify how the java

should be converted. Java

s are normally converted to

since this is the most common usage of strings. Strings are automatically converted to a

-terminated array of

across the function call. Returned

values are automatically copied into a

if the method signature returns

(

, for example).

If the native method returns char* and actually allocates memory, a return type of Pointer should be used to avoid leaking the memory. It is then up to you to take the necessary steps to free the allocated memory.

When converting Java unicode characters into an array of char, the default platform encoding is used, unless the system property jna.encoding is set to a valid encoding. This property may be set to "UTF8", for example, to ensure all native strings use that encoding.

Arrays of String passed to native code (either as a function argument or callback return value) will be converted into a NULL-terminated array of char* (or wchar_t* in the case of an array of WString.

The

class is used to identify wide character strings. Unicode values are copied directly from the Java

array to a native

array.

Use arrays to represent buffers of primitive types passed to a function for use only during the function invocation. If the native code keeps a pointer to the memory after the native function returns, use direct

s or

instead.

A native method cannot return a Java array, since there is no canonical way to indicate the intended length of the returned array. Instead, use one of the array access methods in the Pointer class, supplying the length of the returned array.

Buffers may also be used as a memory buffer input argument; direct byte buffers can often provide much improved performance over primitive arrays. A pointer provided by native code may be converted to a Buffer by calling Pointer.getByteBuffer(long, long).

If you need to pass in a subset of a primitive array, you can do so by wrapping it in a Buffer subclass, such as ByteBuffer, using the ByteBuffer.wrap(byte[],int,int) method. Wrapping an array in a buffer also allows you to pass only a subset of a Java array to the native function.

JNA supports supplying Java callbacks to native code. You must define an interface that extends the

interface, and define a single

method with a signature that matches the function pointer required by the native code. The name of the method may be something other than "callback" only if there is only a single method in the interface which extends Callback or the class which implements

. The arguments and return value follow the same rules as for a direct function invocation.

When accessing Windows APIs, sometimes the documentation indicates that a function pointer parameter must refer to a function that resides in a DLL. In these instances, add the DLLCallback interface to your callback definition. The function pointer as seen by Windows will be located in the jnidispatch.dll module.

If the callback returns a String or String[], the returned memory will be valid until the returned object is GC'd.

If your native code initializes function pointers within a struct, JNA will automatically generate a Callback instance matching the declared type. This enables you to easily call the function supplied by native code using proper Java syntax.

// Original C code
struct _functions {
  int (*open)(const char*,int);
  int (*close)(int);
};

// Equivalent JNA mapping
public class Functions extends Structure {
  public static interface OpenFunc extends Callback {
    int invoke(String name, int options);
  }
  public static interface CloseFunc extends Callback {
    int invoke(int fd);
  }
  public OpenFunc open;
  public CloseFunc close;
}
...
Functions funcs = new Functions();
lib.init(funcs);
int fd = funcs.open.invoke("myfile", 0);
funcs.close.invoke(fd);

Callbacks may also be used as return values. Native function pointers are wrapped in a proxy implementing the declared Callback type, to facilitate calling from Java.

// Original C code
typedef void (*sig_t)(int);
sig_t signal(int signal, sig_t sigfunc);

// Equivalent JNA mapping
public interface CLibrary extends Library {
    public interface SignalFunction extends Callback {
        void invoke(int signal);
    }
    SignalFunction signal(int signal, SignalFunction func);
}

If you need control over the thread context in which a

operates, you can install a

for any given callback object. The first time the callback is called on a thread that is not currently attached to the VM, the initializer will be queried to determine how the thread should be set up. You can indicate the desired name, thread group, and daemon state for the thread, as well as indicating whether the thread should be left attached to the VM after callback exit. The latter improves performance if you know you will be getting multiple callbacks on the same thread, avoiding the need for the VM to generate multiple Java Thread objects for the same native thread. If you do leave the native thread attached, you should either ensure you detach it at some later point (by calling

from within the callback just prior to return) or return true from your

method so that the native thread will not prevent the VM from exiting.

If you don't need to otherwise customize the callback thread, you can simply call

from within your callback to indicate whether the thread attachment should be maintained or not.

The C varargs function definition may be mapped to a Java varargs method definition. For example,

// Original C code
extern int printf(const char* fmt, ...);

// Equivalent JNA mapping
interface CLibrary extends Library {
    int printf(String fmt, ...);
}

The Java

represents a native

. By default, this type is treated as a pointer to structure (

) on the native side when used as a parameter or return value. When used as a structure field, the structure is interpreted as by value. To force the complementary interpretation, the tagging interfaces

and

are provided.

The data within a Java Structure is automatically written to native memory just before a native function call with a struct parameter, and automatically read from native memory after the function returns.

To pass a pointer to a structure as an argument, simply use the Java structure subclass, and a pointer to native data memory will be used. The contents of the structure will be passed to the function and updated when the function returns. Structures are packed according to the default alignment rules for the platform's native C

s.

// Original C code
typedef struct _Point {
  int x, y;
} Point;

Point* translate(Point* pt, int dx, int dy);

// Equivalent JNA mapping
class Point extends Structure { public int x, y; }
Point translate(Point pt, int x, int y);
...
Point pt = new Point();
Point result = translate(pt, 100, 100);

To pass a structure by value, first define the structure, then define an empty class from that which implements

. Use the

class as the argument or return type.

// Original C code
typedef struct _Point {
  int x, y;
} Point;

Point translate(Point pt, int dx, int dy);

// Equivalent JNA mapping
class Point extends Structure {
    public static class ByValue extends Point implements Structure.ByValue { }
    public int x, y;
}
Point.ByValue translate(Point.ByValue pt, int x, int y);
...
Point.ByValue pt = new Point.ByValue();
Point result = translate(pt, 100, 100);

To pass an array of structures, simply use a Java array of the desired structure type. If the array is uninitialized, it will be auto-initialized prior to the function call.

// Original C code
void get_devices(struct Device[], int size);

// Equivalent JNA mapping
int size = ...
Device[] devices = new Device[size];
lib.get_devices(devices, devices.length);

Alternatively, you can reallocate a single Structure instance into an array as follows:

Device dev = new Device();
// As an array of Structure
Structure[] structs = dev.toArray(size);
// As an array of Device
Device[] devices = (Device[])dev.toArray(size);

Declare the method as returning a

of the appropriate type, then invoke

to convert to an array of initialized structures of the appropriate size. Note that your

class must have a no-args constructor, and you are responsible for freeing the returned memory if applicable in whatever way is appropriate for the called function.

// Original C code
struct Display* get_displays(int* pcount);
void free_displays(struct Display* displays);

// Equivalent JNA mapping
Display get_displays(IntByReference pcount);
void free_displays(Display[] displays);
...
IntByReference pcount = new IntByReference();
Display d = lib.get_displays(pcount);
Display[] displays = (Display[])d.toArray(pcount.getValue());
...
lib.free_displays(displays);

Nested structures are treated as consecutive memory (as opposed to pointers to structures). For example:

// Original C code
typedef struct _Point {
  int x, y;
} Point;

typedef struct _Line {
  Point start;
  Point end;
} Line;

// Equivalent JNA mapping
class Point extends Structure {
  public int x, y;
}

class Line extends Structure {
  public Point start;
  public Point end;
}

Explicit initialization of nested structures is not required; the objects will be created as needed and properly mapped to the parent structure's memory.

If you need a pointer to a structure within your structure, you can use the Structure.ByReference tagging interface to indicate the field should be treated as a pointer instead of inlining the full structure.

// Original C code
typedef struct _Line2 {
  Point* p1;
  Point* p2;
} Line2;

// Equivalent JNA mapping
class Point extends Structure {
    public static class ByReference extends Point implements Structure.ByReference { }
    public int x, y;
}
class Line2 extends Structure {
  public Point.ByReference p1;
  public Point.ByReference p2;
}

The more general case is just a pointer to memory. This allows you to define the field without necessarily defining the inner structure itself, similar to declaring a struct without defining it in C:

// Original C code
typedef struct _Line2 {
  Point* p1;
  Point* p2;
} Line2;

// Equivalent JNA mapping
class Line2 extends Structure {
  public Pointer p1;
  public Pointer p2;
}

Line2 line2;
Point p1, p2;
...
line2.p1 = p1.getPointer();
line2.p2 = p2.getPointer();

Structures with nested arrays require an explicit constructor to ensure the structure size is properly calculated.

typedef struct _Buffer {
  char buf1[32];
  char buf2[1024];
} Buffer;

class Buffer extends Structure {
  public byte[] buf1 = new byte[32];
  public byte[] buf2 = new byte[1024];
}

Calculation of the native size of the structure is deferred until the structure is actually used.

Structures with variable size, or with primitive array elements, for example:

// Original C code
typedef struct _Header {
  int flags;
  int buf_length;
  char buffer[1];
} Header;

require a constructor which establishes the required size for the structure and initializes things appropriately. For example:

// Equivalent JNA mapping
class Header extends Structure {
  public int flags;
  public int buf_length;
  public byte[] buffer;
  public Header(int bufferSize) {
    buffer = new byte[bufferSize];
    buf_length = buffer.length;
    allocateMemory();
  }
}

Normally, JNA will write the entire contents of a

prior to a function call and read back from native memory after the function call. Sometimes a structure field is not intended for client use, gets modified asynchronously by hardware, or otherwise is effectively read-only. If you expect any fields of the structure to be modified by any agent outside your Java program, you should mark the field

. This prevents JNA from automatically updating the native memory from the Java value. You can still force an update of the native memory from the Java value by calling

for the field in question.

class Data extends com.sun.jna.Structure {
  public volatile int refCount;
  public int value;
}
...
Data data = new Data();

In the above example, the field

will only be written to native memory based on the Java value with a call to

. To obtain the current state of native memory, call

(to update the entire structure) or

(to update just the

field).

If you want to absolutely prevent Java code from modifying a

's contents, you may mark its fields

. Structure reads can still overwrite the values based on native memory contents, but no Java code will be able to modify any of the fields.

class ReadOnly extends com.sun.jna.Structure {
  // Do not initialize the field here, or the compiler will inline the value!
  public final int refCount;
  {
    // Initialize fields here, to ensure the values are not inlined
    refCount = -1;
    read();
    // refCount might now have a different value
  }
}
...
ReadOnly ro = new ReadOnly();
// Will not compile!
ro.refCount = 0;

Make certain you attend to the following:

1. All final fields should be initialized in the constructor.
2. If you call Structure.read() from anywhere but the constructor, keep in mind that the compiler and/or hotspot will be assuming field values will not change across that function call.

Unions are a special type of Structure. Each declared field within the union overlays the same space in native memory. When writing a union to native memory, you

specify which field is to be written by supplying the desired field's class to the

method. On read, all non-pointer-based fields will be initialized from native memory. Structure, String, and WString members will

be initialized unless they are selected via

.

If a function sets the system error property (

or

), the error code will be thrown as a

if you declare the exception in your JNA mapping. Alternatively, you can use

to retrieve it. Throwing an exception is preferred since it has better performance.

In some cases, such as invoking native VM functions directly, it is necessary to pass Java objects to the native methods. By default, JNA disallows using any Java object that is not explicitly supported unless it derives from

, because it is generally unnecessary to use such objects and usually signals a programmer error. To avoid errors flagging the use of Java objects, use the library load option

with Boolean.TRUE.

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Sometimes native functions exist only as C preprocessor macros or as inline functions. If you need to do more than simply change the name of the invoked function (which can be handled via

), an

allows you to arbitrarily reconfigure the function invocation, including changing the method name and reordering, adding, or removing arguments. See the

documentation for details.

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The method

may be used to obtain the address of global variables as a

. Pointer methods may then be used to read or write the value as appropriate for the variable type.

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It is not uncommon when defining a new library and writing tests to encounter memory access errors which crash the VM. These are often caused by improper mappings or invalid arguments passed to the native library. To generate Java errors instead of crashing the VM, call

. Not all platforms support this protection; if not, the value of

will remain

.

NOTE: When protected mode is enabled, you should make use of the jsig library, if available (see Signal Chaining) to avoid interfering with the JVM's use of signals. In short, set the environment variable LD_PRELOAD (or LD_PRELOAD_64) to the path to libjsig.so in your JRE lib directory (usually ${java.home}/lib/${os.arch}/libjsig.so) before launching your Java application.

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Using

of methods makes native calls more efficiently than does interface mapping. Direct mapping does not support varargs calls or arrays of Pointer, String, or WString as an argument or return value. For optimium results, use only primitive arguments and return values; you'll have to convert to and from objects yourself explicitly.

Type mapping incurs additional overhead on each function call. You can avoid this by ensuring that your arguments and/or return types are already primitive types.

Java primitive arrays are generally slower to use than direct memory (Pointer, Memory, or ByReference) or NIO buffers, since the Java memory has to be pinned and possibly copied across the native call, since the Java array is not necessarily contiguously allocated.

Structures are normally written to native memory before and read back from native memory after a function call. With very large structures, there can be a performance hit using reflection to walk through all the fields. Structure auto-synch can be disabled by calling

with a

parameter. It is then up to you to use

and

or

to synch with just the fields of interest.

In those methods where you are interested in the value of errno/GetLastError(), declare your method to throw

.

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