Errata

Dear Mr. Darwin,

first of all I would like to say that I like your book very much and I am using it frequently.

During my current project, I used it to step in to thread programming. After some programming, I think I have found a mistake in recipe 24.6 on page 702.

In the first code example, the lock.lock() method call is put into the try block. If an Exception is thrown from this method call, the lock.unlock() method call in the finally block will be executed anyway, which may lead to an IllegalMonitorStateException for example.

The API (http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/Lock.html) shows the lock() method call outside the try block, which solves this problem:

Lock l = ...;
l.lock();
try {
// access the resource protected by this lock
} finally {
l.unlock();
}

I have marked this as a 'Serious technical mistake' because most times it is likely covered by overlying Exceptions and because it is hard to reproduce in a debugging environment.

If I missed something, please give me a short hint about it.

Please excuse my English, as it is not my native tongue.

With kind regards,

Christian Binder

Note from the Author or Editor:
Thanks for reporting this. I think I would classify it as a "minor technical mistake" because, in fact, the lock() method is unlikely to throw exceptions. However, I have changed it as you suggest in the online source code, and will change it in subsequent editions of the book; Publications can change it if they reprint the book meanwhile.

The fragment at the top of the page should change as follows:
(Pubs: print on wide paper, and interpret as "diff -u" output):

diff -u -r1.1 -r1.2
--- LockFragment.java 1 Apr 2005 19:03:09 -0000 1.1
+++ LockFragment.java 15 Nov 2008 15:42:38 -0000 1.2
@@ -4,11 +4,12 @@

public class LockFragment {

+ Lock theLock = null;
+
/** This file exists to hold the skeleton code at the top of Recipe 24.6 */
public void demo() {
- Lock theLock = null;
+ theLock.lock();
try {
- theLock.lock();
// do the work that is protected by the Lock
} finally {
theLock.unlock();


The code example, ReadersWriterDemo.java, should be changed as follows:

diff -u -r1.7 -r1.8
--- ReadersWriterDemo.java 10 Feb 2007 16:24:02 -0000 1.7
+++ ReadersWriterDemo.java 15 Nov 2008 15:42:38 -0000 1.8

@@ -45,16 +47,23 @@
for (int i = 0; i < NUM_READER_THREADS; i++) {
new Thread() {
public void run() {
- while(!done) {
- Iterator results = null;
+ while (!done) {
+ Iterator<BallotPosition> results = null;
+ lock.readLock().lock();
try {
- lock.readLock().lock();
results = theData.iterator();
} finally {
// Unlock in finally to be sure it gets done.
lock.readLock().unlock();
}

@@ -68,17 +77,19 @@
// Start one writer thread to simulate occasional voting
new Thread() {
public void run() {
- while(!done) {
+ while (!done) {
+ lock.writeLock().lock();
try {
- lock.writeLock().lock();
theData.voteFor(
- (((int)(Math.random()*
- theData.getCandidateCount()))));
+ // Vote for random candidate :-)
+ // Performance: should have one PRNG per thread.
+ (((int)(Math.random()*
+ theData.getCandidateCount()))));
} finally {
lock.writeLock().unlock();
}
try {
- Thread.sleep(((long)(Math.random()*1500)));
+ Thread.sleep(((long)(Math.random()*1000)));
} catch (InterruptedException ex) {
// nothing to do
}

There's a compiler error when trying to compile 9780596001704/RE/BookRank.java:
unclosed string literal:
public final static String QUERY = "
"http://www.quickbookshops.web/cgi-bin/search?isbn=";
The quote on line 21 should be removed:
public final static String QUERY =
"http://www.quickbookshops.web/cgi-bin/search?isbn=";