In the System.java source, the standard input, output and error streams are declared final and initialized null?

Question

public final static InputStream in = null;
public final static PrintStream out = null;
public final static PrintStream err = null;

But as we very w

Answer 1

They are later on set by native methods SetIn0, SetOut0 and SetErr0

private static native void setIn0(InputStream in);
private static native void setOut0(PrintStream out);
private static native void setErr0(PrintStream err);

called from the initializeSystemClass method, which according to the JavaDoc is called after thread initialization.

FileInputStream fdIn = new FileInputStream(FileDescriptor.in);
FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);
FileOutputStream fdErr = new FileOutputStream(FileDescriptor.err);
setIn0(new BufferedInputStream(fdIn));
setOut0(new PrintStream(new BufferedOutputStream(fdOut, 128), true));
setErr0(new PrintStream(new BufferedOutputStream(fdErr, 128), true));

Answer 2

final fields are not necessarily constant. They can still be manipulated, it's just that manipulation is only prevented at compile-time, specifically by preventing you from using the assignment operator (=). See this question and JLS §17.5.3, specifically:

final fields can be changed via reflection and other implementation-dependent means.

This is necessary for things like deserialization. This can also cause some interesting caveats since compilers can optimize final fields on compile-time and run-time. The JLS linked above has an example of this.

Answer 3

This is done in order to prevent "hacking". These fields can be changed only by appropriate setters that call native methods

private static native void setIn0(InputStream in);
private static native void setOut0(PrintStream out);
private static native void setErr0(PrintStream err);

Native methods can do everything including changing final fields.