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MemoryFile


android.os

Class MemoryFile



  • public class MemoryFile
    extends Object
    MemoryFile is a wrapper for the Linux ashmem driver. MemoryFiles are backed by shared memory, which can be optionally set to be purgeable. Purgeable files may have their contents reclaimed by the kernel in low memory conditions (only if allowPurging is set to true). After a file is purged, attempts to read or write the file will cause an IOException to be thrown.
    • Constructor Summary

      Constructors
      Constructor and Description
      MemoryFile(String name, int length)
      Allocates a new ashmem region.
    • Constructor Detail

      • MemoryFile

        public MemoryFile(String name,
                  int length)
                   throws IOException
        Allocates a new ashmem region. The region is initially not purgable.
        Parameters:
        name - optional name for the file (can be null).
        length - of the memory file in bytes.
        Throws:
        IOException - if the memory file could not be created.
    • Method Detail

      • close

        public void close()
        Closes the memory file. If there are no other open references to the memory file, it will be deleted.
      • finalize

        protected void finalize()
        Description copied from class: Object
        Invoked when the garbage collector has detected that this instance is no longer reachable. The default implementation does nothing, but this method can be overridden to free resources.

        Note that objects that override finalize are significantly more expensive than objects that don't. Finalizers may be run a long time after the object is no longer reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup. Note also that finalizers are run on a single VM-wide finalizer thread, so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary for a class that has a native peer and needs to call a native method to destroy that peer. Even then, it's better to provide an explicit close method (and implement Closeable), and insist that callers manually dispose of instances. This works well for something like files, but less well for something like a BigInteger where typical calling code would have to deal with lots of temporaries. Unfortunately, code that creates lots of temporaries is the worst kind of code from the point of view of the single finalizer thread.

        If you must use finalizers, consider at least providing your own ReferenceQueue and having your own thread process that queue.

        Unlike constructors, finalizers are not automatically chained. You are responsible for calling super.finalize() yourself.

        Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer thread. See Effective Java Item 7, "Avoid finalizers" for more.

        Overrides:
        finalize in class Object
      • length

        public int length()
        Returns the length of the memory file.
        Returns:
        file length.
      • isPurgingAllowed

        public boolean isPurgingAllowed()
        Is memory file purging enabled?
        Returns:
        true if the file may be purged.
      • allowPurging

        public boolean allowPurging(boolean allowPurging)
                             throws IOException
        Enables or disables purging of the memory file.
        Parameters:
        allowPurging - true if the operating system can purge the contents of the file in low memory situations
        Returns:
        previous value of allowPurging
        Throws:
        IOException
      • getInputStream

        public InputStream getInputStream()
        Creates a new InputStream for reading from the memory file.
        Returns:
        InputStream
      • getOutputStream

        public OutputStream getOutputStream()
        Creates a new OutputStream for writing to the memory file.
        Returns:
        OutputStream
      • readBytes

        public int readBytes(byte[] buffer,
                    int srcOffset,
                    int destOffset,
                    int count)
                      throws IOException
        Reads bytes from the memory file. Will throw an IOException if the file has been purged.
        Parameters:
        buffer - byte array to read bytes into.
        srcOffset - offset into the memory file to read from.
        destOffset - offset into the byte array buffer to read into.
        count - number of bytes to read.
        Returns:
        number of bytes read.
        Throws:
        IOException - if the memory file has been purged or deactivated.
      • writeBytes

        public void writeBytes(byte[] buffer,
                      int srcOffset,
                      int destOffset,
                      int count)
                        throws IOException
        Write bytes to the memory file. Will throw an IOException if the file has been purged.
        Parameters:
        buffer - byte array to write bytes from.
        srcOffset - offset into the byte array buffer to write from.
        destOffset - offset into the memory file to write to.
        count - number of bytes to write.
        Throws:
        IOException - if the memory file has been purged or deactivated.
      • getFileDescriptor

        public FileDescriptor getFileDescriptor()
                                         throws IOException
        Gets a FileDescriptor for the memory file. The returned file descriptor is not duplicated.
        Throws:
        IOException - If the memory file has been closed.
      • getSize

        public static int getSize(FileDescriptor fd)
                           throws IOException
        Returns the size of the memory file that the file descriptor refers to, or -1 if the file descriptor does not refer to a memory file.
        Throws:
        IOException - If fd is not a valid file descriptor.


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