Android对于I/O操作效率比较低，提高效率的方法可以通过NDK直接进行读写，或者调用MemoryFile.java类对I/O进行处理。
一、MemoryFile.java 源码分析
MemoryFile.java 源码位置：frameworks/base/core/java/android/os/MemoryFile.java
MemoryFile类直接对native方法进行封装，通过JNI调用底层C++方法，下面对具体函数进行分析。
1. 构造函数
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;
}
}
这里直接调用native方法native_mmap对name文件进行内存映射，这里的native_mmap方法调用JNI方法android_os_MemoryFile_open，这个函数又通过运行库提供的ashmem_create_region来创建内存共享映射，这个函数的实现在system/core/libcutils/ashmem-dev.c中，具体代码如下：
/*
* ashmem_create_region – creates a new ashmem region and returns the
file
* descriptor, or <0 on error
*
* `name’ is an optional label to give the region (visible in
/proc/pid/maps)
* `size’ is the size of the region, in page-aligned bytes
*/
int ashmem_create_region(const char *name, size_t size)
{
int fd, ret;
fd = open(ASHMEM_DEVICE, O_RDWR); // open函数打开ASHMEM_DEVICE设备文件
if (fd < 0)
return fd;
if (name) {
char buf[ASHMEM_NAME_LEN];
strlcpy(buf, name, sizeof(buf));
ret = ioctl(fd, ASHMEM_SET_NAME, buf);
if (ret < 0)
goto error;
}
ret = ioctl(fd, ASHMEM_SET_SIZE, size); // 设置匿名共享内存的名称和大小
if (ret < 0)
goto error;
return fd;
error:
close(fd);
return ret;
}
调用native_mmap函数对打开的设备文件进行映射，调用JNI函数android_os_MemoryFile_mmap，这个函数利用内存映射mmap实现。
2. readBytes 函数
具体源码如下：
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);
}
readBytes函数同样调用JNI接口native_read即android_os_MemoryFile_read，通过SetByteArrayRegion进行内存的读取。
3. writeBytes函数
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);
}
这里调用native_write接口，通过android_os_MemoryFile_write 调用GetByteArrayRegion对内存进行写操作。
这两个函数用到了ashmem_pin_region和ashmem_unpin_region是系统运行时库提供的接口，用来执行我们前面说的匿名共享内存的锁定和解锁操作，它们的作用是告诉Ashmem驱动程序，它的哪些内存块是正在使用的，需要锁定，哪些内存是不需要使用了，可以它解锁，这样，Ashmem驱动程序就可以辅助内存管理系统来有效地管理内存了。
二、MemoryFile.java 使用实例
public class MemoryFileTest {
private Context content;
public MemoryFileTest(Context content) {
this.content = content;
}
public void testFileDescriptor() throws Exception {
MemoryFile file = new MemoryFile(“MemoryFileTest”, 1000000);
byte[] buffer = new byte[testString.length];
// check low level accessors
file.writeBytes(testString, 0, 2000, testString.length);
file.readBytes(buffer, 2000, 0, testString.length);
compareBuffers(testString, buffer, testString.length);
// check streams
buffer = new byte[testString.length];
OutputStream os = file.getOutputStream();
os.write(testString);
InputStream is = file.getInputStream();
is.mark(testString.length);
is.read(buffer);
compareBuffers(testString, buffer, testString.length);
// test mark/reset
buffer = new byte[testString.length];
is.reset();
is.read(buffer);
compareBuffers(testString, buffer, testString.length);
file.close();
}
private void compareBuffers(byte[] buffer1, byte[] buffer2, int length) throws Exception {
for (int i = 0; i < length; i++) {
if (buffer1[i] != buffer2[i]) {
Toast.makeText(content, “readBytes did not read back what writeBytes wrote”, Toast.LENGTH_SHORT).show();
}
}
}
private static final byte[] testString = new byte[] {
3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9, 3, 2, 3, 8, 4, 6, 2, 6, 4, 3, 3, 8, 3, 2, 7, 9, 5, 0, 2, 8, 8, 4, 1, 9, 7, 1, 6, 9, 3, 9, 9, 3, 7, 5, 1, 0, 5, 8, 2, 0, 9, 7, 4, 9, 4, 4, 5, 9, 2, 3, 0, 7, 8, 1, 6, 4,
0, 6, 2, 8, 6, 2, 0, 8, 9, 9, 8, 6, 2, 8, 0, 3, 4, 8, 2, 5, 3, 4, 2, 1, 1, 7, 0, 6, 7, 9, 8, 2, 1, 4, 8, 0, 8, 6, 5, 1, 3, 2, 8, 2, 3, 0, 6, 6, 4, 7, 0, 9, 3, 8, 4, 4, 6, 0, 9, 5, 5, 0, 5, 8, 2, 2, 3, 1, 7, 2,
5, 3, 5, 9, 4, 0, 8, 1, 2, 8, 4, 8, 1, 1, 1, 7, 4, 5, 0, 2, 8, 4, 1, 0, 2, 7, 0, 1, 9, 3, 8, 5, 2, 1, 1, 0, 5, 5, 5, 9, 6, 4, 4, 6, 2, 2, 9, 4, 8, 9, 5, 4, 9, 3, 0, 3, 8, 1, 9, 6, 4, 4, 2, 8, 8, 1, 0, 9, 7, 5,
6, 6, 5, 9, 3, 3, 4, 4, 6, 1, 2, 8, 4, 7, 5, 6, 4, 8, 2, 3, 3, 7, 8, 6, 7, 8, 3, 1, 6, 5, 2, 7, 1, 2, 0, 1, 9, 0, 9, 1, 4, 5, 6, 4, 8, 5, 6, 6, 9, 2, 3, 4, 6, 0, 3, 4, 8, 6, 1, 0, 4, 5, 4, 3, 2, 6, 6, 4, 8, 2,
1, 3, 3, 9, 3, 6, 0, 7, 2, 6, 0, 2, 4, 9, 1, 4, 1, 2, 7, 3, 7, 2, 4, 5, 8, 7, 0, 0, 6, 6, 0, 6, 3, 1, 5, 5, 8, 8, 1, 7, 4, 8, 8, 1, 5, 2, 0, 9, 2, 0, 9, 6, 2, 8, 2, 9, 2, 5, 4, 0, 9, 1, 7, 1, 5, 3, 6, 4, 3, 6,
7, 8, 9, 2, 5, 9, 0, 3, 6, 0, 0, 1, 1, 3, 3, 0, 5, 3, 0, 5, 4, 8, 8, 2, 0, 4, 6, 6, 5, 2, 1, 3, 8, 4, 1, 4, 6, 9, 5, 1, 9, 4, 1, 5, 1, 1, 6, 0, 9, 4, 3, 3, 0, 5, 7, 2, 7, 0, 3, 6, 5, 7, 5, 9, 5, 9, 1, 9, 5, 3,
0, 9, 2, 1, 8, 6, 1, 1, 7, 3, 8, 1, 9, 3, 2, 6, 1, 1, 7, 9, 3, 1, 0, 5, 1, 1, 8, 5, 4, 8, 0, 7, 4, 4, 6, 2, 3, 7, 9, 9, 6, 2, 7, 4, 9, 5, 6, 7, 3, 5, 1, 8, 8, 5, 7, 5, 2, 7, 2, 4, 8, 9, 1, 2, 2, 7, 9, 3, 8, 1,
8, 3, 0, 1, 1, 9, 4, 9, 1, 2, 9, 8, 3, 3, 6, 7, 3, 3, 6, 2, 4, 4, 0, 6, 5, 6, 6, 4, 3, 0, 8, 6, 0, 2, 1, 3, 9, 4, 9, 4, 6, 3, 9, 5, 2, 2, 4, 7, 3, 7, 1, 9, 0, 7, 0, 2, 1, 7, 9, 8, 6, 0, 9, 4, 3, 7, 0, 2, 7, 7,
0, 5, 3, 9, 2, 1, 7, 1, 7, 6, 2, 9, 3, 1, 7, 6, 7, 5, 2, 3, 8, 4, 6, 7, 4, 8, 1, 8, 4, 6, 7, 6, 6, 9, 4, 0, 5, 1, 3, 2, 0, 0, 0, 5, 6, 8, 1, 2, 7, 1, 4, 5, 2, 6, 3, 5, 6, 0, 8, 2, 7, 7, 8, 5, 7, 7, 1, 3, 4, 2,
7, 5, 7, 7, 8, 9, 6, 0, 9, 1, 7, 3, 6, 3, 7, 1, 7, 8, 7, 2, 1, 4, 6, 8, 4, 4, 0, 9, 0, 1, 2, 2, 4, 9, 5, 3, 4, 3, 0, 1, 4, 6, 5, 4, 9, 5, 8, 5, 3, 7, 1, 0, 5, 0, 7, 9, 2, 2, 7, 9, 6, 8, 9, 2, 5, 8, 9, 2, 3, 5,
4, 2, 0, 1, 9, 9, 5, 6, 1, 1, 2, 1, 2, 9, 0, 2, 1, 9, 6, 0, 8, 6, 4, 0, 3, 4, 4, 1, 8, 1, 5, 9, 8, 1, 3, 6, 2, 9, 7, 7, 4, 7, 7, 1, 3, 0, 9, 9, 6, 0, 5, 1, 8, 7, 0, 7, 2, 1, 1, 3, 4, 9, 9, 9, 9, 9, 9, 8, 3, 7,
2, 9, 7, 8, 0, 4, 9, 9, 5, 1, 0, 5, 9, 7, 3, 1, 7, 3, 2, 8, 1, 6, 0, 9, 6, 3, 1, 8, 5, 9, 5, 0, 2, 4, 4, 5, 9, 4, 5, 5, 3, 4, 6, 9, 0, 8, 3, 0, 2, 6, 4, 2, 5, 2, 2, 3, 0, 8, 2, 5, 3, 3, 4, 4, 6, 8, 5, 0, 3, 5,
2, 6, 1, 9, 3, 1, 1, 8, 8, 1, 7, 1, 0, 1, 0, 0, 0, 3, 1, 3, 7, 8, 3, 8, 7, 5, 2, 8, 8, 6, 5, 8, 7, 5, 3, 3, 2, 0, 8, 3, 8, 1, 4, 2, 0, 6, 1, 7, 1, 7, 7, 6, 6, 9, 1, 4, 7, 3, 0, 3, 5, 9, 8, 2, 5, 3, 4, 9, 0, 4,
2, 8, 7, 5, 5, 4, 6, 8, 7, 3, 1, 1, 5, 9, 5, 6, 2, 8, 6, 3, 8, 8, 2, 3, 5, 3, 7, 8, 7, 5, 9, 3, 7, 5, 1, 9, 5, 7, 7, 8, 1, 8, 5, 7, 7, 8, 0, 5, 3, 2, 1, 7, 1, 2, 2, 6, 8, 0, 6, 6, 1, 3, 0, 0, 1, 9, 2, 7, 8, 7,
6, 6, 1, 1, 1, 9, 5, 9, 0, 9, 2, 1, 6, 4, 2, 0, 1, 9, 8, 9, 3, 8, 0, 9, 5, 2, 5, 7, 2, 0, 1, 0, 6, 5, 4, 8, 5, 8, 6, 3, 2, 7, 8, 8, 6, 5, 9, 3, 6, 1, 5, 3, 3, 8, 1, 8, 2, 7, 9, 6, 8, 2, 3, 0, 3, 0, 1, 9, 5, 2,
0, 3, 5, 3, 0, 1, 8, 5, 2, 9, 6, 8, 9, 9, 5, 7, 7, 3, 6, 2, 2, 5, 9, 9, 4, 1, 3, 8, 9, 1, 2, 4, 9, 7, 2, 1, 7, 7, 5, 2, 8, 3, 4, 7, 9, 1, 3, 1, 5, 1, 5, 5, 7, 4, 8, 5, 7, 2, 4, 2, 4, 5, 4, 1, 5, 0, 6, 9, 5, 9,
5, 0, 8, 2, 9, 5, 3, 3, 1}
}
