Java 17 Recipes

 Solution 2
Using a reentrant lock, you can prevent multiple threads from accessing the same 
critical area of the code. In this solution, the inventoryLock is acquired by calling 
inventoryLock.lock(). Any other thread that tries to acquire the inventoryLock lock 
must wait until the inventoryLock lock is released. At the end of the fulfillOrder() 
method (in the finally block), the inventoryLock is released by calling the 
inventoryLock.unlock() method.
Lock inventoryLock = new ReentrantLock();
private boolean fulfillOrder(String itemOrdered, int quantityOrdered, 
String customerName) {
try {
inventoryLock.lock();
int currentInventory = inventoryMap.get(itemOrdered);
if (currentInventory < quantityOrdered) {
System.out.println("Couldn't fulfill order for " + 
customerName +
" not enough " + itemOrdered + " (" + 
quantityOrdered + ")");
return false; // sorry, we sold out
}
inventoryMap.put(itemOrdered, currentInventory - quantityOrdered);
CustomerOrder order = new CustomerOrder(itemOrdered, 
quantityOrdered, customerName);
customerOrders.add(order);
System.out.println("Order fulfilled for " + customerName + " of " +
itemOrdered + " (" + quantityOrdered + ")");
return true;
} finally {
inventoryLock.unlock();
}
}
Chapter 10 ConCurrenCy

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private void checkInventoryLevels() {
try {
inventoryLock.lock();
System.out.println("------------------------------------");
inventoryMap.entrySet().stream().forEach((inventoryEntry) -> {
System.out.println("Inventory Level :" + inventoryEntry 
.getKey() + " " + inventoryEntry.getValue());
});
System.out.println("------------------------------------");
} finally {
inventoryLock.unlock();
}
}
private void displayOrders() {
try {
inventoryLock.lock();
customerOrders.stream().forEach((order) -> {
System.out.println(order.getQuantityOrdered() + " " +
order.getItemOrdered() + " for " + order.getCustomerName());
});
} finally {
inventoryLock.unlock();
}
}