Question

I was looking through the list of Java bytecode instructions and I noticed that there aren’t any I/O instructions. That intrigued me. How does the JVM execute methods like System.out.println when it doesn’t support I/O instructions?

If it uses some form of memory mapped I/O then how does it communicate with the OS to read file descriptors, etc? Does the JVM implement its own layer of abstraction for handling I/O operations? Are the Java I/O packages (java.io & java.nio) implemented in C/C++ instead?

Answer 1

If you look at the library source code, you’ll see that all interfacing with low-level APIs (OS etc) is done using native code.

For example, take FileOutputStream:

/**
 * Opens a file, with the specified name, for writing.
 * @param name name of file to be opened
 */
private native void open(String name) throws FileNotFoundException;

/**
 * Writes the specified byte to this file output stream. Implements
 * the <code>write</code> method of <code>OutputStream</code>.
 *
 * @param      b   the byte to be written.
 * @exception  IOException  if an I/O error occurs.
 */
public native void write(int b) throws IOException;

/**
 * Writes a sub array as a sequence of bytes.
 * @param b the data to be written
 * @param off the start offset in the data
 * @param len the number of bytes that are written
 * @exception IOException If an I/O error has occurred.
 */
private native void writeBytes(byte b[], int off, int len) throws IOException;

And then there’s the corresponding C file (often OS-specific).