# The Java Memory Model — Advanced Java

Source: https://www.geekswithgeeks.com/en/advanced-java/adv-jmm

> Learn the happens-before relation — the only guarantee that one thread sees another's writes — and why unsynchronized code can be reordered by the compiler and CPU.

## happens-before

The JMM only guarantees one thread sees another's write if a **happens-before** edge links them: unlocking then locking the same monitor, writing then reading the same `volatile`, `Thread.start()`, `Thread.join()`, and constructor-to-final-field reads. Without such an edge, the compiler and CPU may reorder freely.

## Broken double-checked locking

Without `volatile`, a reader thread can observe a partially-constructed `instance` because the JMM allows the constructor write and the field write to be reordered. `volatile` adds the happens-before edge that fixes it.

```java
class Config {
    private static volatile Config instance;   // volatile is required
    static Config get() {
        if (instance == null) {
            synchronized (Config.class) {
                if (instance == null) instance = new Config();
            }
        }
        return instance;
    }
}
```