happens-before
- Java course
- Expert
- Lesson 8/46
- 5 min
Learn memory ordering with happens-before.
The happens-before relation
happens-before is the ordering guarantee of the Java Memory Model (JMM). If action A happens-before B, then the memory effects of A are visible to B. This gives consistency between threads.
Several mechanisms establish it: a synchronized block makes the changes of the thread releasing the lock visible to the one acquiring it; a volatile write is seen by later reads; final fields become visible when the constructor finishes. Without happens-before a thread may never see another's write.
Ordering with synchronized
A synchronized block or method forces all threads using the same lock to enter one at a time. All changes made by the thread releasing the lock become visible to the next thread acquiring it.
Example: in a Counter class with synchronized void inc() and synchronized int get(), when a thread calls inc and releases the lock, a later thread calling get sees the new value. This is the most concrete form of the happens-before relation.
Practice Synchronized counter
Write a Counter class with a private int count field, synchronized void inc() and synchronized int get() methods. inc does count++. In main, call inc twice and print the get result with System.out.println.
Starter code
// Define the Counter class
// Add synchronized inc and get
// Print the result in mainSample solution
class Counter {
private int count;
public synchronized void inc() { count++; }
public synchronized int get() { return count; }
}
public class HappensBeforeDemo {
public static void main(String[] args) {
Counter c = new Counter();
c.inc();
c.inc();
System.out.println(c.get());
}
}Expected output
2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the guarantee mechanisms
| Term | Meaning |
|---|---|
synchronized |
Happens-before |
volatile |
Visibility |
final |
Safe publication |
local variable |
Single thread |
2 Which mechanism establishes the happens-before relation?
- synchronized block Correct answer
- local variable
- final field
- Metaspace
Explanation: synchronized establishes happens-before with a lock.
3 Add the synchronized keyword
public ____ void inc() { count++; }public synchronized void inc() { count++; }Explanation: synchronized locks and orders the method.