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Threading: Lesson

Previously, we learned how to block thread execution using `join()`. However, this is only to block thread execution from the context where the thread was started. For example, if a thread was created and started in the main thread, we can only call `join()` on it from the main thread and wait on its completion from there.

In Java, to control thread execution from within other threads, we can use the `wait()`, `notify()`, and `notifyAll()` methods. These are primarily used to protect shared resources from being used by two threads at the same time or to wait until some condition has changed in a thread.
 
When using `wait()` and `notifyAll()`, it is important to do so in a `synchronized(this)` block. When we create a `synchronized(this)` block, we are telling Java that we want it to be the only thread accessing the fields of the class at a given moment. In the `synchronized(this)` block, we must:
 
1. Check the condition on which to wait.
2. Decide whether to `wait()` (block the execution of the current thread) or `notifyAll()` (allow other threads to check their condition again and proceed)

<details>
<summary>Take a look at this simple example</summary>
 
```java
import java.lang.Thread;
 
public class OrderDinnerProcess {
 private boolean foodArrived = false;
 
 private void printTask(String task) { 
   System.out.println(Thread.currentThread().getName() + " - " + task);
 }
 
 public void eatFood() {
   printTask("Wow, I am starving!");
   try {
     synchronized (this) {
       while (!this.foodArrived) {
         printTask("Waiting for the food to arrive...");
         wait();
       }
     }
   } catch (InterruptedException e) {
     System.out.println(e);
   }
   printTask("Finally! Yum yum yum!!!");
 }
 
 public void deliverFood() {
   printTask("Driving food over...");
   try {
     Thread.sleep(5000);
     synchronized (this) {
       this.foodArrived = true;
       printTask("Arrived!");
       notifyAll();
     }
   } catch (InterruptedException e) {
     System.out.println(e);
   }
 }
 
 public static void main(String[] args) {
   OrderDinnerProcess p = new OrderDinnerProcess();
   try {
     for (int i = 0; i < 5; i++) {
       Thread eatFood = new Thread(() -> p.eatFood());
       eatFood.start();
     }
     Thread.sleep(1000);
     Thread delivery = new Thread(() -> p.deliverFood());
     delivery.start();
   } catch (InterruptedException e) {
     System.out.println(e);
   }
 }
}
```
</details>

In the example, we are simulating a family of five eager to eat some dinner! However, they can't eat until the food has arrived. We could have controlled this behavior before by defining, starting, and joining the `delivery` thread before the five `eatFood()` threads began. However, using `wait()` and `notifyAll()`, we can start them in any order, and each `eatFood()` thread will `wait()` until a call to `notifyAll()` by another thread tells them to proceed to check again.
 
The `synchronized` block is used here to tell Java that only one thread should be able to read from and write to the `foodArrived()` field at a time.
 
The `wait()` function is used to pause execution for a thread until a call has been made to `notifyAll()`, which triggers another check of the condition.
 
The `notifyAll()` function is used to tell all threads that are currently waiting that they may now proceed.
 
In addition to being used to coordinate thread execution, we can use the `synchronized(this)` block, `wait()`, and `notifyAll()` to control access to shared resources. If a resource is currently in use, a thread should `wait()` until a call to `notifyAll()` is made, which indicates that the shared resource _may_ have been released and is ready for use.
 
In this exercise, you'll use the `synchronized(this)` block, `wait()`, and `notifyAll()` to update our `CakeMaker` class so that only one thread can use the mixing bowl at a given time. The other threads must `wait()` until the mixing bowl is released, and are instructed to continue with execution with a call to `notifyAll()`.

Communicating Between Threads

Another important scenario when working with multi-threaded programs is _managing shared resources_ between threads.
 
When we access the same data from two different threads, we may cause a _race condition_. A race condition occurs when some inconsistency is caused by two threads trying to access the same shared data at the same time. Consider this example where two threads are attempting to increment a value simultaneously:

```Java
class IntegerMapper{
  public int[] array = {1, 2, 3, 4, 5};    
  public void incrementElement(int i, int j){
    array[i] += j;
  }   
}
public class Main{
  public static void main(String args[]) throws InterruptedException{
    IntegerMapper iMapper = new IntegerMapper();
    Thread thread1 = new Thread(() -> {
      for(int i = 0; i < 100; i++){
        iMapper.incrementElement(2, 4);
        try{
          Thread.sleep(10);
        }
        catch(InterruptedException exception){
          System.out.println("Error!");
        }
      }
            
    });
    Thread thread2 = new Thread(() -> {
      for(int i = 0; i < 100; i++){
        iMapper.incrementElement(2, 3);
        try{
          Thread.sleep(10);
        }
        catch(InterruptedException exception){
          System.out.println("Error!");
        }
      }
    });
        
    thread1.start();
    thread2.start();
        
    thread1.join();
    thread2.join();
        
    System.out.println(iMapper.array[2]);
  }
}
``` 
If we run this program multiple times, we will get a different output each time! This is because `thread1` and `thread2` conflict in their access of `array[2]`. To update the value in the `incrementElement()` method, the value must be read, incremented, then saved. One thread could be reading the value while the other thread is incrementing it, leading to inconsistencies. 

We can prevent race conditions on shared data by using the `synchronized` keyword in Java. In a threaded program, when we add `synchronized` to the definition of a function, it will ensure that for a given instance of a class, only one thread can run that method at a time. We can fix `incrementElement()` by making it `synchronized` like so:

```java
public synchronized void incrementElement(int i, int j){
    array[i] += j;
}
```
Now, each time the program is run, it will output the same value.
  
Let's see how we can use thread synchronization to solve a classic race condition problem.

Thread Synchronization

Congratulations on completing this lesson on threading! So far, you have:
 
- created threads in three different ways:
 - extending the `Thread` class
 - implementing the `Runnable` class
 - using the in-line, lambda expression syntax
- started threads with `.start()`
- paused the execution of threads using `Thread.sleep(long millis)`
- observed thread execution using the _supervisor pattern_
- blocked on thread completion with `.join()`
- synchronized access to shared resources from threads with the `synchronized` keyword
- communicated between threads with `.wait()` and `.notify()`
 
That's a lot to have covered!
 
You can now use threading to solve many intermediate and advanced problems when programming with Java. Try to think of more examples like the `FortuneTeller` or `CakeMaker` classes, that help to perform multiple tasks at the same time.


Review

So far, you have seen how to start threads with the `start()` method and learned multiple ways to implement threads into sequential programs.
 
Sometimes, we want to be able to see the status of threads during their execution. In Java, the best pattern for doing so is to use a **supervisor thread**. This is a pattern where the main thread (or another thread) is able to watch and check on the progress of another thread, as long as it has access to the corresponding `Thread` instance. This kind of thread is implemented just like other threads. Supervisor threads are often used for updating the user of our program on the progress of an ongoing task.
 
One `Thread` method that a supervisor thread may use to monitor another thread is `isAlive()`. This method returns `true` if the thread is still running, and `false` if it has terminated. A supervisor might continuously poll this value (check it at a fixed interval) until it changes, and then notify the user that the thread has changed state. Here is an example:
 
```java
import java.time.Instant;
import java.time.Duration;
 
public class Factorial{
 public int compute(int n){
   // the existing method to compute factorials
 }
 
 // utility method to create a supervisor thread
 public static Thread createSupervisor(Thread t){
   Thread supervisor = new Thread(() -> {
     Instant startTime = Instant.now();
     // supervisor is polling for t's status
     while (t.isAlive()) {
       System.out.println(Thread.currentThread().getName() + " - Computation still running...");
       Thread.sleep(1000);
     }
   });
 
   // setting a custom name for the supervisor thread
   supervisor.setName("supervisor");
   return supervisor;
 
 }
 
 public static void main(String[] args){
   Factorial f = new Factorial();
 
   Thread t1 = new Thread(() -> {
     System.out.println("25 factorial is...");
     System.out.println(f.compute(25));
   });
 
 
   Thread supervisor = createSupervisor(t1);
 
   t1.start();
   supervisor.start();
 
   System.out.println("Supervisor " + supervisor.getName() + " watching worker " + t1.getName());
 }
}
```
 
The thread labeled `supervisor` here is polling the status of `t1` (a "worker" thread) continuously at an interval of 1000 milliseconds (one second). The supervisor checks the status of `t1` using the `isAlive()` method in a `while` loop.
 
Additionally, the `getName()` method will return a unique name for a thread in the current context. This comes in handy when debugging multi-threaded programs: the programmer can better understand which thread is performing a certain task at a given moment. We can also name a thread ourselves, using the `setName()` method of the `Thread` class.

Now, let's create a supervisor thread that polls for the status of _all_ of the running `CrystalBall` threads.

Supervising a Thread

So far, we have learned about threads and how they work in theory. Before we put it into practice, we should have a strong understanding of what kinds of problems multi-threaded programs are best suited to solve.
 
Threading can solve problems where there is a lot of waiting to be done. When one task is in a "waiting" or "blocked" state, another one can start. This is useful for programs in which a long-running computation needs to be done, or when a task must wait on a response from an external source such as fetching an image from a server.
 
In the following exercise, you'll see how a program could be forced to "wait" for a task's completion. As you complete it, start thinking about what tasks could be performed in threads, concurrently, to save some time. 

Introduction

In the previous exercise, you learned that we can convert a class to run as a thread by extending the `Thread` class. We can also create threaded classes in Java by implementing the `Runnable` interface.
 
This approach is preferred because we are only allowed to extend one class, and wasting it on `Thread` might not be beneficial to our program. Here, rather than extending the capability of the built-in `Thread` class, we just want to use its threading capability. Because of this, implementing the `Runnable` interface (which is what the `Thread` class does anyway) and passing in the object into a new `Thread` object is the preferred way of implementing multithreading. Here's an example of how we would implement a `Factorial` thread by implementing `Runnable` instead of extending `Thread`:
 
```java
public class Factorial implements Runnable {
 private int n;
 
 public Factorial(int n) {
   this.n = n;
 }
 
 public int compute(int n) {
   // ... the code to compute factorials
 }
 
 public void run() {
   System.out.print("Factorial of " + String.valueOf(this.n) + ":")
   System.out.println(this.compute(this.n));
 }
 
 public static void main(String[] args) {
   Factorial f = new Factorial(25);
   Factorial g = new Factorial(10);
   Thread t1 = new Thread(f);
   Thread t2 = new Thread(f);
   t1.start();
   t2.start();
 }
}
```
 
A more succinct way of using the `Runnable` interface is to use _lambda expressions_. This is a more modern syntax that allows us to define the `run()` method we want to use inline without requiring the class to implement `Runnable` or extend `Thread`. For the above example, the syntax looks like this:
 
```java
public class Factorial {
 public int compute(int n) {
   // ... the code to compute factorials
 }
 
 public static void main(String[] args) {
   Factorial f = new Factorial();
 
   // the lambda function replacing the run method
   new Thread(() -> {
     System.out.println(f.compute(25));
   }).start();
 
   // the lambda function replacing the run method
   new Thread(() -> {
     System.out.println(f.compute(10));
   }).start();
 }
}
```
 
Behind the scenes, this syntax is still using the `Runnable` interface. This is because Java will translate this _lambda_ syntax into something like this:
 
```java
new Thread(new Runnable() {
 void run() {
   System.out.println(f.compute(25));
 }
}).start();
```
 
Using the lambda expression syntax for starting threads, we can create a threaded version of our class in only a few lines!
 
This syntax has several benefits:
 
- Our class no longer needs to extend `Thread` OR implement `Runnable`. We can make a thread from anything!
- Our class no longer needs a constructor to store arguments! Since we can pass an argument directly into the compute function when we create our thread, we no longer need to create a separate instance of our object any time we want to perform a threaded task with it.
- Our class is easier to read! The lambda syntax makes it so that people reading our code can immediately identify what task is being performed in our thread without having to read our class first to find the `run()` method.
 
Now, let's try both of these methods of implementing `Runnable` with our `FortuneTeller` class.

**Note:** Sometimes you will see developers specifically import the Thread and Runnable classes from `java.lang`. This is not necessary since all Java programs naturally import the `java.lang` package anyway. It is often used to help with readability or remind an author to add a specific feature.

Implementing the Runnable Interface

There are multiple ways to implement [threading](https://www.codecademy.com/resources/docs/java/threading?page_ref=catalog) in Java. The first that we will try involves extending the `Thread` class. In Java, there is a built-in class that handles threads: `java.lang.Thread`. To create a thread, we create a class that extends the built-in `Thread` class and then overrides its `public void run()` method. Beyond these two requirements, the rest of our class can have anything we want in it!

The `public void run()` method that we override performs the actual work that our thread should be responsible for. This may be performing a long-running computation, calling out to another application or service, or other tasks that involve some waiting.

Notice that the `run()` method is a `void` and accepts no parameters. Every `run()` method must be of this signature. If `run()` requires some external information, that information must be a field declared in the class and initialized in the constructor. However, be very careful of using external information as this may present some synchronization issues.

When we defined our class in this way, we can then create an instance of our class in any other part of our program and call `start()` on the instance to trigger the thread’s execution.

Here's an example of a sequential program in Java that computes the meaning of life:

```java
public class HugeProblemSolver{

  public HugeProblemSolver(){
    // Required constructor used for passing information to the start() method.
  }

  private static void solveComputation(){
    // Solves random computation
    // Takes anywhere from 1 second to 10 minutes
    System.out.println("The answer is: 42");
  }

  public static void main(String[] args){
    HugeProblemSolver.solveComputation();
    HugeProblemSolver.solveComputation();
  }
}
```

In this program, we will only see the answer to the second question _**after**_ the first question has been answered. A threaded solution would allow us to start the computations at the same time, and receive the answers as soon as they are available.

Let's take a look at the threaded approach:

```java
public class HugeProblemSolver extends Thread{

  public HugeProblemSolver(){
    // Required constructor used for passing information to the start() method.
  }

  private static void solveComputation(){
    // Solves random computation
    // Takes anywhere from 1 second to 10 minutes
  }

  @Override
  public void run(){
    solveComputation();
    System.out.println("The answer is: 42");
  }

  public static void main(String[] args){
    HugeProblemSolver m1 = new HugeProblemSolver();
    HugeProblemSolver m2 = new HugeProblemSolver();
    m1.start();
    m2.start();
  }
}
```

Now, you can see that all that changed was:

- Extended the `Thread` Class.
- Created and Overrode the `run()` method from `Thread`.
- Instantiated `HugeProblemSolver` and called `start()` which starts a new thread and searches in the class for the `run()` method to execute.

Now, both `Thread`s we created are working on solving their own problems simultaneously. Whichever one finishes first will print to the console first. There is no need to wait on sequential order.

Let's see how we can implement this in our `CrystalBall` class.

Extending the Thread Class

Another common scenario in multi-threaded programs is to wait for a thread to complete before proceeding in a path of execution.
 
In other languages, this concept is called "awaiting" or "blocking". In Java, we say that we wait for the thread to **join**.
 
When a thread joins, it means that the thread's task is complete and the process that was initially forked off has been "joined" back into the main thread.
 
This is useful when we can only perform a specific task once several prerequisite tasks have been completed. Take a look at **BurgerMaker.java** in the editor, which describes the process of preparing a cheeseburger. A cheeseburger is generally prepared in the following way:

1. Grill hamburger patty.
2. Once the patty is properly cooked, add cheese on top to melt it.
3. Toast the burger buns.
4. Fry onions.
5. Once all ingredients are ready, assemble the burger.

Many of these steps can be done concurrently. However, notice that some of the steps are dependent on the completion of other steps. Step 2 requires the patty to be grilled before melting cheese on top of it. Step 5 requires all other steps to be completed before it can start (we can't assemble a burger until everything is ready).     

In `BurgerMaker`, we can see how `join()` is used to enforce dependencies between different tasks. By using `start()` and `join()` properly, we can ensure that tasks that can be performed concurrently are started. Still, tasks with dependencies are not started until all dependencies have been joined.
 
In this exercise, we'll use the `join()` features we see in **BurgerMaker.java** to ensure that tasks occur in the correct order while making a cake in our new file, **CakeMaker.java**. `CakeMaker` describes baking a cake by following these steps:
A cake is made in the following way:
   1. The oven must be preheated before the cake can be baked.
   2. The dry ingredients and wet ingredients should be mixed before the ingredients can be combined.
   3. The ingredients must be combined before the cake can be baked.
   4. The cake must be finished baking before the cake can be frosted.