package com.ximple.io.dgn7;
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//~--- JDK imports ------------------------------------------------------------
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import java.io.IOException;
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import java.util.HashMap;
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import java.util.Map;
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import org.apache.log4j.LogManager;
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import org.apache.log4j.Logger;
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/**
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* Lock
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*
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* @author Ulysses
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* @version 0.1
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* @since 2006/5/18
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*/
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public class Lock {
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Logger logger = LogManager.getLogger("com.ximple.io.dgn7");
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/**
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* indicates a write is occurring
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*/
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int writeLocks = 0;
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/**
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* if not null a writer is waiting for the lock or is writing.
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*/
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Thread writer;
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/**
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* Thread->Owner map. If empty no read locks exist.
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*/
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Map owners = new HashMap();
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/**
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* If the lock can be read locked the lock will be read and default
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* visibility for tests
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*
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* @return
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* @throws java.io.IOException
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*/
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synchronized boolean canRead() throws IOException {
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if ((writer != null) && (writer != Thread.currentThread())) {
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return false;
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}
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if (writer == null) {
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return true;
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}
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if (owners.size() > 1) {
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return false;
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}
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return true;
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}
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/**
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* If the lock can be read locked the lock will be read and default
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* visibility for tests
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*
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* @return
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* @throws IOException
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*/
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synchronized boolean canWrite() throws IOException {
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if (owners.size() > 1) {
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return false;
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}
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if ((canRead()) && ((writer == Thread.currentThread()) || (writer == null))) {
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if (owners.isEmpty()) {
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return true;
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}
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if (owners.containsKey(Thread.currentThread())) {
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return true;
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}
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}
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return false;
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}
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/**
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* Called by shapefileReader before a read is started and before an IOStream
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* is openned.
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*
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* @throws IOException
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*/
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public synchronized void lockRead() throws IOException {
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if (!canRead()) {
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while ((writeLocks > 0) || (writer != null)) {
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try {
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wait();
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} catch (InterruptedException e) {
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throw (IOException) new IOException().initCause(e);
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}
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}
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}
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assertTrue("A write lock exists that is owned by another thread", canRead());
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Thread current = Thread.currentThread();
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Owner owner = (Owner) owners.get(current);
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if (owner != null) {
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owner.timesLocked++;
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} else {
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owner = new Owner(current);
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owners.put(current, owner);
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}
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logger.debug("Start Read Lock:" + owner);
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}
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private void assertTrue(String message, boolean b) {
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if (!b) {
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throw new AssertionError(message);
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}
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}
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/**
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* Called by ShapefileReader after a read is complete and after the IOStream
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* is closed.
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*/
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public synchronized void unlockRead() {
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assertTrue("Current thread does not have a readLock", owners.containsKey(Thread.currentThread()));
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Owner owner = (Owner) owners.get(Thread.currentThread());
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assertTrue("Current thread has " + owner.timesLocked + "negative number of locks", owner.timesLocked > 0);
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owner.timesLocked--;
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if (owner.timesLocked == 0) {
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owners.remove(Thread.currentThread());
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}
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notifyAll();
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logger.debug("unlock Read:" + owner);
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}
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/**
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* Called by ShapefileDataStore before a write is started and before an
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* IOStream is openned.
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*
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* @throws IOException
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*/
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public synchronized void lockWrite() throws IOException {
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Thread currentThread = Thread.currentThread();
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if (writer == null) {
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writer = currentThread;
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}
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while (!canWrite()) {
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try {
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wait();
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} catch (InterruptedException e) {
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throw (IOException) new IOException().initCause(e);
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}
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if (writer == null) {
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writer = currentThread;
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}
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}
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if (writer == null) {
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writer = currentThread;
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}
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assertTrue("The current thread is not the writer", writer == currentThread);
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assertTrue("There are read locks not belonging to the current thread.", canRead());
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writeLocks++;
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logger.debug(currentThread.getName() + " is getting write lock:" + writeLocks);
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}
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/**
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* default visibility for tests
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*/
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synchronized int getReadLocks(Thread thread) {
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Owner owner = (Owner) owners.get(thread);
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if (owner == null) {
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return -1;
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}
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return owner.timesLocked;
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}
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public synchronized void unlockWrite() {
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if (writeLocks > 0) {
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assertTrue("current thread does not own the write lock", writer == Thread.currentThread());
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assertTrue("writeLock has already been unlocked", writeLocks > 0);
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writeLocks--;
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if (writeLocks == 0) {
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writer = null;
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}
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}
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logger.debug("unlock write:" + Thread.currentThread().getName());
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notifyAll();
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}
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/**
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* default visibility for tests
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*/
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synchronized boolean ownWriteLock(Thread thread) {
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return (writer == thread) && (writeLocks > 0);
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}
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private class Owner {
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final Thread owner;
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int timesLocked;
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Owner(Thread owner) {
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this.owner = owner;
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timesLocked = 1;
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}
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public String toString() {
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return owner.getName() + " has " + timesLocked + " locks";
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}
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}
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}
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