关于System.in的分析

Question

最近在看《JAVA编程思想》，其中System.in迷惑了我。
首先看下System的源码(jdk1.6)
[code="java"]
public final static InputStream in = nullInputStream();

private static InputStream nullInputStream() throws NullPointerException {
if (currentTimeMillis() > 0) {
return null;
}
throw new NullPointerException();
}
[/code]

疑问1： 通过源码可以看到 System.in是System类的一个静态属性，其为InputStream，初始时通过nullInputStream()方法返回，但是return null？

疑问2： 写了一个测试类，通过语句System.out.println(System.in); 打印出结果为：java.io.BufferedInputStream@1fb8ee3，说明System.in是BufferedInputStream类型，如何实现的？

疑问3： 如果线程执行到System.in.read();时，未在console输入内容，线程将挂起，直到console有输入内容。如何实现的？

Answer 1

[code="java"]
/**

• The following two methods exist because in, out, and err must be
• initialized to null. The compiler, however, cannot be permitted to
  • inline access to them, since they are later set to more sensible values
• by initializeSystemClass(). */ private static InputStream nullInputStream() throws NullPointerException { if (currentTimeMillis() > 0) return null; throw new NullPointerException(); } [/code]

注意上面的注释：initializeSystemClass().
[code="java"]
private static void initializeSystemClass() {
props = new Properties();
initProperties(props);
sun.misc.Version.init();
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));
[/code]
这里的in 不就是个buffer类型的了。咋都不看方法的注释呢！~~