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Java FileInputStream

Jakob Jenkov
Last update: 2019-08-28

The Java FileInputStream class, java.io.FileInputStream, makes it possible to read the contents of a file as a stream of bytes. The Java FileInputStream class is a subclass of Java InputStream. This means that you use the Java FileInputStream as an InputStream (FileInputStream behaves like an InputStream).

Java FileInputStream Example

Here is a simple FileInputStream example:

InputStream input = new FileInputStream("c:\\data\\input-text.txt");

int data = input.read();
while(data != -1) {
  //do something with data...
  doSomethingWithData(data);

  data = input.read();
}
input.close();

Note: The proper exception handling has been skipped here for the sake of clarity. To learn more about correct exception handling, go to Java IO Exception Handling.

Note also, that since FileInputStream is a subclass of InputStream, we can cast the created FileInputStream to an InputStream everywhere we want to, as we do in the example above.

FileInputStream Constructors

The FileInputStream class has a three different constructors you can use to create a FileInputStream instance. I will cover the first two here.

The first constructor takes a String as parameter. This String should contain the path in the file system to where the file to read is located. Here is a code example:

String path = "C:\\user\\data\\thefile.txt";

FileInputStream fileInputStream = new FileInputStream(path);

Notice the path String. It needs double backslashes (\\) to create a single backslash in the String, because backslash is an escape character in Java Strings. To get a single backslash you need to use the escape sequence \\.

On unix the file path could have looked like this:

String path = "/home/jakobjenkov/data/thefile.txt";

Notice the use of the for-slash (the normal slash character) as directory separator. That is how you write file paths on unix. Actually, in my experience Java will also understand if you use a / as directory separator on Windows (e.g. c:/user/data/thefile.txt), but don't take my word for it. Test it on your own system!

The second FileInputStream constructor takes a File object as parameter. The File object has to point to the file you want to read. Here is an example:

String path = "C:\\user\\data\\thefile.txt";
File   file = new File(path);

FileInputStream fileInputStream = new FileInputStream(file);

Which of the constructors you should use depends on what form you have the path in before opening the FileInputStream. If you already have a String or File, just use that as it is. There is no particular gain in converting a String to a File, or a File to a String first.

read()

The read() method of a FileInputStream returns an int which contains the byte value of the byte read. If the read() method returns -1, there is no more data to read in the FileInputStream, and it can be closed. That is, -1 as int value, not -1 as byte value. There is a difference here!

You use the read() method just like the read() method of an InputStream. Here is an example of reading all data in a Java FileInputStream :

FileInputStream fileInputStream =
    new FileInputStream("c:\\data\\input-text.txt");


int data = fileInputStream.read();
while(data != -1) {
    // do something with data variable

    data = fileInputStream.read(); // read next byte
}

When the while loop terminates, all bytes have been read from the FileInputStream.

read(byte[])

Being an InputStream the FileInputStream also has two read() methods which can read data into a byte array. These methods are inherited from the Java InputStream class, by the way. These methods are:

  • int read(byte[])
  • int read(byte[], int offset, int length)

The first method attempts to fill up the byte array passed as parameter to it with bytes from from the FileInputStream.

The second method attempts to read length bytes into the byte array, starting from cell offset in the byte array, and filling forward from there.

Both methods return the number of bytes actually read into the byte array. In case there are less bytes to read than what there is space for in the array, or less than you specified in the length parameter, less bytes will be read into the byte array. If all bytes have been read from the FileInputStream, these read() methods will return -1. Therefore it is necessary to inspect the value returned from these read() method calls.

Here is an example of calling one of the read(byte[]) methods:

FileInputStream fileInputStream = new FileInputStream("c:\\data\\input-text.txt");

byte[] data      = new byte[1024];
int    bytesRead = fileInputStream.read(data, 0, data.length);

while(bytesRead != -1) {
  doSomethingWithData(data, bytesRead);

  bytesRead = fileInputStream.read(data, 0, data.length);
}

Notice that read(data, 0, data.length) is equivalent to read(data) .

The doSomethingWithData() method implementation has been left out of this example to keep it short. But - it represents any set of actions you want to carry out on the read data.

Read Performance

Reading an array of bytes at a time is faster than reading a single byte at a time from a Java FileInputStream. The difference can easily be a factor 10 or more in performance increase, by reading an array of bytes rather than reading a single byte at a time.

The exact speedup gained depends on the size of the byte array you read, and the OS, hardware etc. of the computer you are running the code on. You should study the hard disk buffer sizes etc. of the target system before deciding. However buffer sizes of 8KB and up will give a good speedup. However, once your byte array exceeds the capacity of the underlying OS and hardware, you won't get a bigger speedup from a bigger byte array.

You will probably have to experiment with different byte array size and measure read performance, to find the optimal byte array size.

Transparent Buffering via BufferedInputStream

You can add transparent, automatic reading and buffering of an array of bytes from a FileInputStream using a Java BufferedInputStream . The BufferedInputStream reads a chunk of bytes into a byte array from the underlying FileInputStream. You can then read the bytes one by one from the BufferedInputStream and still get a lot of the speedup that comes from reading an array of bytes rather than one byte at a time. Here is an example of wrapping a Java FileInputStream in a BufferedInputStream :

InputStream input = new BufferedInputStream(
                      new FileInputStream("c:\\data\\input-file.txt"),
                        1024 * 1024        /* buffer size */
    );

Notice, that a BufferedInputStream is an InputStream subclass and can be used in any place where an InputStream can be used.

Close a FileInputStream

When you are finished reading data from a Java FileInputStream you must close it. You close a FileInputStream by calling the close() method inherited from InputStream. Here is an example of opening a FileInputStream, reading all data from it, and then closing it:

FileInputStream fileInputStream = new FileInputStream("c:\\data\\input-text.txt");

int data = fileInputStream.read();
while(data != -1) {
  data = fileInputStream.read();
}
fileInputStream.close();

Notice how the while loop continues until a -1 value is read from the FileInputStream read() method. After that, the while loop exits, and the FileInputStream close() method is called.

The above code is not 100% robust. If an exception is thrown while reading data from the FileInputStream, the close() method is never called. To make the code more robust, you will have to use the Java Java try with resources construct. Proper exception handling for use of Java IO classes is also explained in my tutorial on Java IO Exception Handling.

Here is an example of closing a Java FileInputStream using the try-with-resources construct:

try( FileInputStream fileInputStream = new FileInputStream("file.txt") ) {

    int data = fileInputStream.read();
    while(data != -1){
        data = fileInputStream.read();
    }
}

Notice that the FileInputStream is declared inside the parentheses after the try keyword. This signals to Java that this FileInputStream is to be managed by the try-with-resources construct.

Once the executing thread exits the try block, the FileInputStream is closed. If an exception is thrown from inside the try block, the exception is caught, the FileInputStream is closed, and then the exception is rethrown. You are thus guaranteed that the FileInputStream is closed, when used inside a try-with-resources block.

Convert FileInputStream to Reader

The Java FileInputStream is a byte based stream of data. As you may know, the Java IO API also has a character based set of input streams called "Readers". You can convert a Java FileInputStream to a Java Reader using the Java InputStreamReader. You can read more about how to use the InputStreamReader by clicking the link in the previous sentence, but here is a quick example of converting a Java FileInputStream to an InputStreamReader:

InputStream inputStream       = new FileInputStream("c:\\data\\input.txt");
Reader      inputStreamReader = new InputStreamReader(inputStream);

Jakob Jenkov

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