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5.4 Java访问接口层详解
分析完匿名共享内存的C++访问接口层后，在本节开始分析其Java访问接口层的实现过程。在Android应用程序框架层中，通过使用接口MemoryFile来封装匿名共享内存文件的创建和使用。接口MemoryFile在文件“frameworks/base/core/java/android/os/MemoryFile.java”中定义，具体实现代码如下所示：
public class MemoryFile
{
private static String TAG = "MemoryFile";
//mmap(2) protection flags from <sys/mman.h>
private static final int PROT_READ = 0x1;
private static final int PROT_WRITE = 0x2;
private static native FileDescriptor native_open(String name, int length) throws IOException;
//returns memory address for ashmem region
private static native int native_mmap(FileDescriptor fd, int length, int mode)
throws IOException;
private static native void native_munmap(int addr, int length) throws IOException;
private static native void native_close(FileDescriptor fd);
private static native int native_read(FileDescriptor fd, int address, byte[] buffer,
int srcOffset, int destOffset, int count, boolean isUnpinned) throws IOException;
private static native void native_write(FileDescriptor fd, int address, byte[] buffer,
int srcOffset, int destOffset, int count, boolean isUnpinned) throws IOException;
private static native void native_pin(FileDescriptor fd, boolean pin) throws IOException;
private static native int native_get_size(FileDescriptor fd) throws IOException;
private FileDescriptor mFD; //ashmem file descriptor
private int mAddress; //address of ashmem memory
private int mLength; //total length of our ashmem region
private boolean mAllowPurging = false; //true if our ashmem region is unpinned
/＊＊
＊ Allocates a new ashmem region. The region is initially not purgable.
＊
＊ @param name optional name for the file (can be null).
＊ @param length of the memory file in bytes.
＊ @throws IOException if the memory file could not be created.
＊/
public MemoryFile(String name, int length) throws IOException {
mLength = length;
mFD = native_open(name, length);
if (length > 0) {
mAddress = native_mmap(mFD, length, PROT_READ | PROT_WRITE);
} else {
mAddress = 0;
}
}
在上述代码中，构造方法MemoryFile以指定的字符串调用了JNI方法native_open，目的是建立一个匿名共享内存文件，这样可以得到一个文件描述符。然后使用这个文件描述符为参数调用JNI方法natvie_mmap，并把匿名共享内存文件映射到进程空间中，这样就可以通过映射得到地址空间的方式直接访问内存数据。
再看JNI函数android_os_MemoryFile_get_size，此函数在文件“frameworks\base\core\jni\android_os_MemoryFile.cpp”中定义，具体实现代码如下所示：
static jint android_os_MemoryFile_get_size(JNIEnv＊ env, jobject clazz,
jobject fileDescriptor) {
int fd = jniGetFDFromFileDescriptor(env, fileDescriptor);
//Use ASHMEM_GET_SIZE to find out if the fd refers to an ashmem region.
//ASHMEM_GET_SIZE should succeed for all ashmem regions, and the kernel
//should return ENOTTY for all other valid file descriptors
int result = ashmem_get_size_region(fd);
if (result < 0) {
if (errno == ENOTTY) {
//ENOTTY means that the ioctl does not apply to this object,
//i.e., it is not an ashmem region.
return (jint) -1;
}
//Some other error, throw exception
jniThrowIOException(env, errno);
return (jint) -1;
}
return (jint) result;
}
再看JNI函数android_os_MemoryFile_open，此函数在文件“frameworks\base\core\jni\android_os_MemoryFile.cpp”中定义，具体实现代码如下所示：
static jobject android_os_MemoryFile_open(JNIEnv＊ env, jobject clazz, jstring name, jint length)
{
const char＊ namestr = (name ? env->GetStringUTFChars(name, NULL) : NULL);
int result = ashmem_create_region(namestr, length);
if (name)
env->ReleaseStringUTFChars(name, namestr);
if (result < 0) {
jniThrowException(env, "java/io/IOException", "ashmem_create_region failed");
return NULL;
}
return jniCreateFileDescriptor(env, result);
}
再看JNI函数android_os_MemoryFile_mmap，此函数在文件“frameworks\base\core\jni\android_os_MemoryFile.cpp”中定义，具体实现代码如下所示：
static jint android_os_MemoryFile_mmap(JNIEnv＊ env, jobject clazz, jobject fileDescriptor,
jint length, jint prot)
{
int fd = jniGetFDFromFileDescriptor(env, fileDescriptor);
jint result = (jint)mmap(NULL, length, prot, MAP_SHARED, fd, 0);
if (! result)
jniThrowException(env, "java/io/IOException", "mmap failed");
return result;
}
在文件“frameworks/base/core/java/android/os/MemoryFile.java”中，再看类MemoryFile的成员函数readBytes，功能是读取某一块匿名共享内存的内容。具体实现代码如下所示：
public int readBytes(byte[] buffer, int srcOffset, int destOffset, int count)
throws IOException {
if (isDeactivated()) {
throw new IOException("Can't read from deactivated memory file.");
}
if (destOffset < 0 || destOffset > buffer.length || count < 0
|| count > buffer.length - destOffset
|| srcOffset < 0 || srcOffset > mLength
|| count > mLength - srcOffset) {
throw new IndexOutOfBoundsException();
}
return native_read(mFD, mAddress, buffer, srcOffset, destOffset, count, mAllowPurging);
}
在文件“frameworks/base/core/java/android/os/MemoryFile.java”中，再看类MemoryFile的成员函数writeBytes，功能是写入某一块匿名共享内存的内容。具体实现代码如下所示：
public void writeBytes(byte[] buffer, int srcOffset, int destOffset, int count)
throws IOException {
if (isDeactivated()) {
throw new IOException("Can't write to deactivated memory file.");
}
if (srcOffset < 0 || srcOffset > buffer.length || count < 0
|| count > buffer.length - srcOffset
|| destOffset < 0 || destOffset > mLength
|| count > mLength - destOffset) {
throw new IndexOutOfBoundsException();
}
native_write(mFD, mAddress, buffer, srcOffset, destOffset, count, mAllowPurging);
}
在文件“frameworks/base/core/java/android/os/MemoryFile.java”中，再看类MemoryFile的成员函数isDeactivated，功能是保证匿名共享内存已经被映射到进程的地址空间中。具体实现代码如下所示：
void deactivate() {
if (! isDeactivated()) {
try {
native_munmap(mAddress, mLength);
mAddress = 0;
} catch (IOException ex) {
Log.e(TAG, ex.toString());
}
}
}
private boolean isDeactivated() {
return mAddress == 0;
}
JNI函数native_read和native_write分别由位于C++层的函数android_os_MemoryFile_read和android_os_MemoryFile_write实现，这两个C++的函数在文件frameworks\base\core\jni\android_os_MemoryFile.cpp中定义，具体实现代码如下所示：
static jint android_os_MemoryFile_read(JNIEnv＊ env, jobject clazz,
jobject fileDescriptor, jint address, jbyteArray buffer, jint srcOffset, jint destOffset,
jint count, jboolean unpinned)
{
int fd = jniGetFDFromFileDescriptor(env, fileDescriptor);
if (unpinned && ashmem_pin_region(fd, 0, 0) == ASHMEM_WAS_PURGED) {
ashmem_unpin_region(fd, 0, 0);
jniThrowException(env, "java/io/IOException", "ashmem region was purged");
return -1;
}
env->SetByteArrayRegion(buffer, destOffset, count, (const jbyte ＊)address + srcOffset);
if (unpinned) {
ashmem_unpin_region(fd, 0, 0);
}
return count;
}
static jint android_os_MemoryFile_write(JNIEnv＊ env, jobject clazz,
jobject fileDescriptor, jint address, jbyteArray buffer, jint srcOffset, jint destOffset,
jint count, jboolean unpinned)
{
int fd = jniGetFDFromFileDescriptor(env, fileDescriptor);
if (unpinned && ashmem_pin_region(fd, 0, 0) == ASHMEM_WAS_PURGED) {
ashmem_unpin_region(fd, 0, 0);
jniThrowException(env, "java/io/IOException", "ashmem region was purged");
return -1;
}
env->GetByteArrayRegion(buffer, srcOffset, count, (jbyte ＊)address + destOffset);
if (unpinned) {
ashmem_unpin_region(fd, 0, 0);
}
return count;
}