使用线程读取 Java 中的图像

Question

I have a question regarding reading images in Java. I am trying to read an image using threads and i was curious whether by doing this: myInputFile = new FileInputStream(myFile); I already read the whole data or not. I already read it in 4 chunks using threads and i am curious whether I just read it twice, once with threads and once with FileInputStream , or what does FileInputStream exactly do. Thanks in advance!

Answer 1

The FileInputStream is not reading your file yet, just by calling it like: myInputFile = new FileInputStream(myFile); .

It basically only gives you a handle to the underlying file and prepares to read from it by opening a connection to that file. Also it runs some basic checks including whether the file exists and if its a proper file and not a directory.

Following is stated in the JavaDocs which you can find here :

Creates a FileInputStream by opening a connection to an actual file, the file named by the File object file in the file system. A new FileDescriptor object is created to represent this file connection.

First, if there is a security manager, its checkRead method is called with the path represented by the file argument as its argument.

If the named file does not exist, is a directory rather than a regular file, or for some other reason cannot be opened for reading then a FileNotFoundException is thrown.

Only by calling the FileInputStream.read methods it starts to read and return the contents of the file.

Thereby the FileInputStream.read() method will only read one single byte of the file and the FileInputStream.read(byte[] b) method will read as many bytes as the size of the byte array b .

Edit:

Because reading a file byte by byte is pretty slow and the usage of the plain FileInputStream.read(byte[] b) method can be a bit cumbersome it's a good practice to use the BufferedInputStream to process files in Java.

It'll read by default the next 8192 bytes of a file and buffer it in-memory for faster access. So the BufferedInputStream.read method will still only return a single byte per call, but in the BufferedInputStream it'll mainly be served from an internal buffer. As long the requested bytes are in this buffer, they'll be served from it. The underlying file will be accessed again only when really needed (-> the requested byte is not in the buffer anymore). This drastically reduces the number of read accesses to the hardware (which in comparison is the slowest operation in this process) and therefore boosts the reading performance a lot.

The initialization looks like this:

InputStream i = new BufferedInputStream(new FileInputStream(myFile));

The handling of it is exactly same as with the 'plain' FileInputStream , since they share the same InputStream interface.