本文探讨了在Android中如何通过JNI进行文件描述符（fd）的相关操作，包括jniGetFDFromFileDescriptor和jniSetFileDescriptorOfFD等函数的使用。这些函数在libnativehelper的JNIHelp.h中声明，对于Android开发者来说，理解和掌握这些函数对于进行底层文件I/O操作至关重要。
关键词：jniGetFDFromFileDescriptor， jniSetFileDescriptorOfFD ， jniCreateFileDescriptor
java_io_FileDescriptor.c文件在android源码 是找不到的。也许是被封装到 库或者其他类型的文件中去了。
但是，在android的网站上可以搜到。
实际上，他们的声明位于 libnativehelper/include/nativehelper/JNIHelp.h 。 这样其他代码就可以使用这些函数了。
eclair/ dalvik/ libcore/ luni/ src/ main/ native/ java_io_FileDescriptor.c
| /* |
| * Licensed to the Apache Software Foundation (ASF) under one or more |
| * contributor license agreements. See the NOTICE file distributed with |
| * this work for additional information regarding copyright ownership. |
| * The ASF licenses this file to You under the Apache License, Version 2.0 |
| * (the "License"); you may not use this file except in compliance with |
| * the License. You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| |
| #include"JNIHelp.h" |
| |
| #include<stdlib.h> |
| #include<unistd.h> |
| #include<sys/types.h> |
| #include<sys/stat.h> |
| #include<fcntl.h> |
| #include<errno.h> |
| #include<assert.h> |
| #include<sys/ioctl.h> |
| |
| /* |
| * These are JNI field IDs for the stuff we're interested in. They're |
| * computed when the class is loaded. |
| */ |
| staticstruct{ |
| jfieldID descriptor; /* int */ |
| jmethodID constructorInt; |
| jmethodID setFD; |
| jclass clazz; |
| }gCachedFields; |
| |
| /* |
| * Internal helper function. |
| * |
| * Get the file descriptor. |
| */ |
| static inlineintgetFd(JNIEnv* env, jobject obj) |
| { |
| return(*env)->GetIntField(env, obj,gCachedFields.descriptor); |
| } |
| |
| /* |
| * Internal helper function. |
| * |
| * Set the file descriptor. |
| */ |
| static inlinevoid setFd(JNIEnv* env, jobject obj, jint value) |
| { |
| (*env)->SetIntField(env, obj, gCachedFields.descriptor, value); |
| } |
| |
| /* |
| * native private static void nativeClassInit() |
| * |
| * Perform one-time initialization. If the class is unloaded and re-loaded, |
| * this will be called again. |
| */ |
| static void nativeClassInit(JNIEnv* env, jclass clazz) |
| { |
| gCachedFields.clazz=(*env)->NewGlobalRef(env, clazz); |
| |
| gCachedFields.descriptor= |
| (*env)->GetFieldID(env, clazz,"descriptor","I"); |
| |
| if(gCachedFields.descriptor== NULL){ |
| jniThrowException(env,"java/lang/NoSuchFieldError","FileDescriptor"); |
| return; |
| } |
| |
| gCachedFields.constructorInt= |
| (*env)->GetMethodID(env, clazz,"<init>","()V"); |
| |
| if(gCachedFields.constructorInt== NULL){ |
| jniThrowException(env,"java/lang/NoSuchMethodError","<init>()V"); |
| return; |
| } |
| } |
| |
| /* |
| * public native void sync() |
| */ |
| static void fd_sync(JNIEnv* env, jobject obj){ |
| int fd= getFd(env, obj); |
| |
| if(fsync(fd)!=0){ |
| /* |
| * If fd is a socket, then fsync(fd) is defined to fail with |
| * errno EINVAL. This isn't actually cause for concern. |
| * TODO: Look into not bothering to call fsync() at all if |
| * we know we are dealing with a socket. |
| */ |
| if(errno!= EINVAL){ |
| jniThrowException(env,"java/io/SyncFailedException",""); |
| } |
| } |
| } |
| |
| /* checks to see if class is inited and inits if needed, returning -1 |
| * on fail and 0 on success |
| */ |
| static int checkClassInit(JNIEnv*env){ |
| if(gCachedFields.clazz== NULL){ |
| /* this should cause the class to be inited and |
| * our static variables to be filled in |
| * |
| * (Note that FindClass just loads the class; it doesn't get |
| * initialized until we try to do something with it.) |
| */ |
| jclass clazz; |
| clazz =(*env)->FindClass(env,"java/io/FileDescriptor"); |
| if(clazz== NULL){ |
| jniThrowException(env,"java/lang/ClassNotFoundException", |
| "java.io.FileDescriptor"); |
| return-1; |
| } |
| |
| jfieldID readWriteId; |
| readWriteId =(*env)->GetStaticFieldID(env, clazz,"in", |
| "Ljava/io/FileDescriptor;"); |
| if(readWriteId== NULL){ |
| jniThrowException(env,"java/lang/NoSuchFieldException", |
| "FileDescriptor.readOnly(Z)"); |
| return-1; |
| } |
| |
| (void)(*env)->GetStaticObjectField(env, clazz, readWriteId); |
| } |
| |
| return0; |
| } |
| |
| |
| /* |
| * For JNIHelp.c |
| * Create a java.io.FileDescriptor given an integer fd |
| */ |
| |
| jobject jniCreateFileDescriptor (JNIEnv*env,int fd){ |
| jobject ret; |
| |
| /* the class may not have been loaded yet */ |
| if(checkClassInit(env)<0){ |
| return NULL; |
| } |
| |
| ret =(*env)->NewObject(env, gCachedFields.clazz, |
| gCachedFields.constructorInt); |
| |
| (*env)->SetIntField(env, ret, gCachedFields.descriptor, fd); |
| |
| return ret; |
| } |
| |
| /* |
| * For JNIHelp.c |
| * Get an int file descriptor from a java.io.FileDescriptor |
| */ |
| |
| int jniGetFDFromFileDescriptor(JNIEnv* env, jobject fileDescriptor){ |
| /* should already be initialized if it's an actual FileDescriptor */ |
| assert(fileDescriptor!= NULL); |
| assert(gCachedFields.clazz!= NULL); |
| |
| return getFd(env, fileDescriptor); |
| } |
| |
| /* |
| * For JNIHelp.c |
| * Set the descriptor of a java.io.FileDescriptor |
| */ |
| |
| voidjniSetFileDescriptorOfFD(JNIEnv* env, jobject fileDescriptor,int value){ |
| /* should already be initialized if it's an actual FileDescriptor */ |
| assert(fileDescriptor!= NULL); |
| assert(gCachedFields.clazz!= NULL); |
| |
| setFd(env, fileDescriptor, value); |
| } |
| |
| /* |
| * JNI registration |
| */ |
| static JNINativeMethodgMethods[]={ |
| /* name, signature, funcPtr */ |
| {"oneTimeInitialization","()V", nativeClassInit}, |
| {"syncImpl", "()V", fd_sync} |
| }; |
| intregister_java_io_FileDescriptor(JNIEnv* env){ |
| return jniRegisterNativeMethods(env,"java/io/FileDescriptor", |
| gMethods, NELEM(gMethods)); |
| } |
其声明是在 JNIHelp.h 中进行的。
/*
* Copyright (C) 2007 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/*
* JNI helper functions.
*
* This file may be included by C or C++ code, which is trouble because jni.h
* uses different typedefs for JNIEnv in each language.
*/
#ifndef _NATIVEHELPER_JNIHELP_H
#define _NATIVEHELPER_JNIHELP_H
#include "jni.h"
#include "utils/Log.h"
#ifndef NELEM
# define NELEM(x) ((int) (sizeof(x) / sizeof((x)[0])))
#endif
#ifdef __cplusplus
extern "C" {
#endif
/*
* Register one or more native methods with a particular class.
*/
int jniRegisterNativeMethods(C_JNIEnv* env, const char* className,
const JNINativeMethod* gMethods, int numMethods);
/*
* Throw an exception with the specified class and an optional message.
* The "className" argument will be passed directly to FindClass, which
* takes strings with slashes (e.g. "java/lang/Object").
*
* Returns 0 on success, nonzero if something failed (e.g. the exception
* class couldn't be found).
*
* Currently aborts the VM if it can't throw the exception.
*/
int jniThrowException(C_JNIEnv* env, const char* className, const char* msg);
/*
* Throw a java.lang.RuntimeException, with an optional message.
*/
int jniThrowRuntimeException(JNIEnv* env, const char* msg);
/*
* Throw a java.io.IOException, generating the message from errno.
*/
int jniThrowIOException(C_JNIEnv* env, int errnum);
/*
* Create a java.io.FileDescriptor given an integer fd
*/
jobject jniCreateFileDescriptor(C_JNIEnv* env, int fd);
/*
* Get an int file descriptor from a java.io.FileDescriptor
*/
int jniGetFDFromFileDescriptor(C_JNIEnv* env, jobject fileDescriptor);
/*
* Set an int file descriptor to a java.io.FileDescriptor
*/
void jniSetFileDescriptorOfFD(C_JNIEnv* env, jobject fileDescriptor, int value);
#ifdef __cplusplus
}
#endif
/*
* For C++ code, we provide inlines that map to the C functions. g++ always
* inlines these, even on non-optimized builds.
*/
#if defined(__cplusplus) && !defined(JNI_FORCE_C)
inline int jniRegisterNativeMethods(JNIEnv* env, const char* className,
const JNINativeMethod* gMethods, int numMethods)
{
return jniRegisterNativeMethods(&env->functions, className, gMethods,
numMethods);
}
inline int jniThrowException(JNIEnv* env, const char* className,
const char* msg)
{
return jniThrowException(&env->functions, className, msg);
}
inline int jniThrowIOException(JNIEnv* env, int errnum)
{
return jniThrowIOException(&env->functions, errnum);
}
inline jobject jniCreateFileDescriptor(JNIEnv* env, int fd)
{
return jniCreateFileDescriptor(&env->functions, fd);
}
inline int jniGetFDFromFileDescriptor(JNIEnv* env, jobject fileDescriptor)
{
return jniGetFDFromFileDescriptor(&env->functions, fileDescriptor);
}
inline void jniSetFileDescriptorOfFD(JNIEnv* env, jobject fileDescriptor,
int value)
{
return jniSetFileDescriptorOfFD(&env->functions, fileDescriptor, value);
}
#endif
#endif /*_NATIVEHELPER_JNIHELP_H*/
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