They're nulled out during close().

• jdk/src/java.base/share/classes/java/io/FilterInputStream.java

  Line 49 in 942b950

  protected volatile InputStream in;

• jdk/src/java.base/share/classes/java/io/BufferedInputStream.java

  Line 94 in 942b950

  protected volatile byte[] buf;

Oddly, I do not see similar behavior in FilterOutputStream and BufferedOutputStream.

[edit: later: google/xplat@0c320ea]