以下是我的计划。始终线程0获取打印机，其他线程不获取它。 有一个打印机对象，我想要多个作业线程来使用打印机。如何使该程序工作，以便所有作业都能获得打印机。对我来说，代码流似乎很好。我正在同一个打印机对象上同步。请帮忙。
package classesTesting;
public class PrinterQueue {
final static Printer printer = new Printer();;
public static void main(String[] args) {
// TODO Auto-generated method stub
System.out.println("In Main");
for (int i = 0; i < 5; i++) {
new Thread(new Jobs(), "Thread - " + i).start();
System.out.println("started " + i + " thread");
}
}
}
class Printer {
private boolean isUsed;
Printer() {
this.isUsed = false;
}
public void setUsed(boolean used) {
this.isUsed = used;
}
public boolean isUsed() {
return this.isUsed;
}
}
class Jobs implements Runnable {
String name;
boolean isDataAvailble;
Jobs() {
this.isDataAvailble = true;
}
public void setNoData(boolean noData) {
this.isDataAvailble = false;
}
@Override
public void run() {
while (isDataAvailble) {
if (PrinterQueue.printer.isUsed()) {
try {
System.out.println(Thread.currentThread()
+ "WAITING FOR PRINTER");
synchronized (PrinterQueue.printer) {
PrinterQueue.printer.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
} else {
synchronized (PrinterQueue.printer) {
System.out.println(Thread.currentThread() + "GOT PRINTER");
PrinterQueue.printer.setUsed(true);
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
PrinterQueue.printer.setUsed(false);
PrinterQueue.printer.notify();
}
}
}
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
嗨，我已经修改了我的程序，先获取锁定然后进行条件检查。即使这样，线程0总是得到打印机。其他线程饿死。
修订计划：
package classesTesting;
public class PrinterQueue {
static Printer printer;
public static void main(String[] args) {
// TODO Auto-generated method stub
System.out.println("In Main");
printer = new Printer();
for (int i = 0; i < 5; i++) {
Jobs j1 = new Jobs();
j1.setPrinter(printer);
Thread t1 = new Thread(j1, "Thread - " + i);
t1.start();
System.out.println("started " + i + " thread");
}
}
}
class Printer {
private boolean isUsed;
Printer() {
this.isUsed = false;
}
public void setUsed(boolean used) {
this.isUsed = used;
}
public boolean isUsed() {
return this.isUsed;
}
}
class Jobs implements Runnable {
String name;
Printer printer;
public Printer getPrinter() {
return printer;
}
public void setPrinter(Printer printer) {
this.printer = printer;
}
boolean isDataAvailble;
Jobs() {
this.isDataAvailble = true;
}
public void setNoData(boolean noData) {
this.isDataAvailble = false;
}
@Override
public void run() {
while (isDataAvailble) {
synchronized (PrinterQueue.printer) {
if (this.printer.isUsed()) {
try {
System.out.println(Thread.currentThread()
+ "WAITING FOR PRINTER");
PrinterQueue.printer.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
else {
System.out.println(Thread.currentThread() + "GOT PRINTER");
PrinterQueue.printer.setUsed(true);
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
PrinterQueue.printer.setUsed(false);
PrinterQueue.printer.notify();
}
}
}
}
}
答案 0 :(得分：1)
我认为你要找的是Condition。您首先需要获得一个锁，然后您可以检查一个条件。虽然那个条件持有线程会睡觉。当条件不再保持时，睡眠线程（或下一个睡眠线程）被唤醒以再次检查条件。
您可以在此处详细了解Condition对象：http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/Condition.html
答案 1 :(得分：1)
如果您希望资源以公平的方式可用于所有线程，那么将ReentrantLock与fair = true参数一起使用会更好。也永远不要依赖于并发方式改变的非易失性变量。这是固定代码：
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class PrinterQueue {
static Printer printer;
public static void main(String[] args) {
System.out.println("In Main");
printer = new Printer();
for (int i = 0; i < 5; i++) {
// I added printer constructor parameter to pass the same printer
// to all the Jobs
new Thread(new Jobs(printer), "Thread - " + i).start();
System.out.println("started " + i + " thread");
}
}
}
class Printer {
// internally printer holds a fair ReentrantLock
Lock lock = new ReentrantLock(true);
// call this to get the printer
public void acquire() {
lock.lock();
}
// call this to release the printer, so it's available for other threads
public void release() {
lock.unlock();
}
}
class Jobs implements Runnable {
// Declare isDataAvailble as volatile as you're going to change it from another thread
volatile boolean isDataAvailble;
private final Printer printer;
// constructor now takes the printer argument
Jobs(Printer printer) {
this.isDataAvailble = true;
this.printer = printer;
}
@Override
public void run() {
try {
while (isDataAvailble) {
System.out.println(Thread.currentThread()
+ "Trying to get the printer");
// get the printer
this.printer.acquire();
try {
System.out.println(Thread.currentThread()
+ "Printer acquired!");
// use it
Thread.sleep(3000);
} finally {
// Release the printer. Better to do it in finally block
// so you will release it even if some unexpected exception occurs
this.printer.release();
}
}
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
答案 2 :(得分：0)
它应该是这样的：
获取打印机：
synchronized (PrinterQueue.printer) {
while (PrinterQueue.printer.isUsed()) {
try {
System.out.println(Thread.currentThread()
+ "WAITING FOR PRINTER");
PrinterQueue.printer.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(Thread.currentThread() + "GOT PRINTER");
PrinterQueue.printer.setUsed(true);
}
使用打印机，根据您的代码Thread.sleep()：
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
释放打印机：
synchronized (PrinterQueue.printer) {
PrinterQueue.printer.setUsed(false);
PrinterQueue.printer.notifyAll();
}
您需要使用while而不是if，并且需要测试您同步的同一个对象。并使用notifyAll()而不是notify().
但我不清楚你需要这个，只需synchronized块。