Java 17 Recipes

10-5. Coordinating Different Collections
 Problem
You need to modify different but related collections at the same time, and you want to 
ensure that no other threads can see these modifications until they have been completed.
 Solution 1
By synchronizing on the principal collection, you can guarantee that collection can be 
updated simultaneously. In the following example, the fulfillOrder method needs to 
check the inventory of the order to be fulfilled. If there is enough inventory to fulfill the 
order, it needs to add the order to the customerOrders list. The fulfillOrder() method 
synchronizes on the inventoryMap map and modifies both the inventoryMap and the 
customerOrders list before finishing the synchronized block.
Set orderingThreads = new HashSet<>();
public static void main(String[] args) throws InterruptedException {
Recipe10_5_1 recipe = new Recipe10_5_1();
recipe.start();
}
final Map inventoryMap = new LinkedHashMap<>();
List customerOrders = new ArrayList<>();
Random random = new Random();
private void start() throws InterruptedException {
loadLoggingConfiguration();
// let's populate our inventory with items.
// at most, we have 20 books
for (int i =0;i < 100;i++) {
inventoryMap.put("Apress Book #"+i,100);
}
// now, let's create ordering threads.
for (int i= 0;i < 20;i++) {
createOrderingThread();
}
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//wait a little
Thread.sleep(100);
// Check on inventory right now
checkInventoryLevels();
// Wait little longer
Thread.sleep(100);
orderingThreads.stream().forEach((thread) -> {
thread.interrupt();
});
Thread.sleep(1000);
// Check inventory levels again
checkInventoryLevels();
// Print the orders.
displayOrders();
}
Logger logger = Logger.getLogger("recipeLogger");
private void createOrderingThread() {
Thread orderingThread = new Thread(() -> {
while (!Thread.interrupted()) {
createRandomOrder();
}
});
orderingThread.start();
orderingThreads.add(orderingThread);
}
private void createRandomOrder() {
String itemOrdered = "Apress Book #"+random.nextInt(100);
int quantityOrdered = random.nextInt(2)+1;
String customerName = "Customer :"+UUID.randomUUID().toString();
fulfillOrder(itemOrdered, quantityOrdered, customerName);
}
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class CustomerOrder {
String itemOrdered;
int quantityOrdered;
String customerName;
CustomerOrder(String itemOrdered, int quantityOrdered, String 
customerName) {
this.itemOrdered = itemOrdered;
this.quantityOrdered = quantityOrdered;
this.customerName = customerName;
}
public String getItemOrdered() {
return itemOrdered;
}
public int getQuantityOrdered() {
return quantityOrdered;
}
public String getCustomerName() {
return customerName;
}
}
private boolean fulfillOrder(String itemOrdered, int quantityOrdered, 
String customerName) {
synchronized (inventoryMap) {
int currentInventory = 0;
if (inventoryMap != null) {
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
}
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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;
}
}
private void checkInventoryLevels() {
synchronized (inventoryMap) {
System.out.println("------------------------------------");
inventoryMap.entrySet().stream().forEach((inventoryEntry) -> {
System.out.println("Inventory Level :"+inventoryEntry 
.getKey()+" "+inventoryEntry.getValue());
});
System.out.println("------------------------------------");
}
}
private void displayOrders() {
synchronized (inventoryMap) {
customerOrders.stream().forEach((order) -> {
System.out.println(order.getQuantityOrdered()+"
"+order.getItemOrdered()+" for "+order.getCustomerName());
});
}
}
The output is similar to:
------------------------------------
Inventory Level :Apress Book #0 100
Inventory Level :Apress Book #1 100
Inventory Level :Apress Book #2 100
Inventory Level :Apress Book #3 100
Inventory Level :Apress Book #4 100
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Inventory Level :Apress Book #5 100
Inventory Level :Apress Book #6 100
Inventory Level :Apress Book #7 100
Inventory Level :Apress Book #8 100
Inventory Level :Apress Book #9 100
Inventory Level :Apress Book #10 100
Inventory Level :Apress Book #11 100
Inventory Level :Apress Book #12 100
Inventory Level :Apress Book #13 100
Inventory Level :Apress Book #14 100
Inventory Level :Apress Book #15 100
...
------------------------------------
Order fulfilled for Customer :70a697d1-08d2-4ad7-b00c-17558930e0ab of 
Apress Book #64 (2)
Order fulfilled for Customer :4db86e58-a895-4992-9090-84a52f9314f6 of 
Apress Book #44 (2)
Order fulfilled for Customer :cc278024-0630-424f-99bb-091d4bb73d25 of 
Apress Book #20 (1)
Order fulfilled for Customer :f43f9edd-17f3-403c-b3b6-b91521b9f970 of 
Apress Book #24 (1)
Order fulfilled for Customer :a1c94246-3671-4373-bbbb-968d32fca100 of 
Apress Book #53 (2)
Order fulfilled for Customer :110382cf-79ff-4042-9ad4-bb4316764281 of 
Apress Book #31 (2)
Order fulfilled for Customer :e3255a74-dfdc-4ef4-b388-02745249c0a9 of 
Apress Book #38 (1)
Order fulfilled for Customer :fd2fa1fd-b9b8-4bca-92a8-6023a1fbde3e of 
Apress Book #40 (2)
Order fulfilled for Customer :5cead5b3-70bf-4f98-82c4-5ee34e90f02d of 
Apress Book #53 (2)
Order fulfilled for Customer :c0e0b2d3-4d58-41ef-9127-8e3d5665d976 of 
Apress Book #38 (1)
Order fulfilled for Customer :38f5ea73-d22b-4714-a319-7599d30a1b9a of 
Apress Book #86 (1)
...
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