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`ThreadLocal`s should be stored in `static` variables to avoid memory leaks. If
a `ThreadLocal` is stored in an instance (non-static) variable, there will be
<code>M \* N</code> instances of the `ThreadLocal` value where `M` is the number
of threads, and `N` is the number of instances of the containing class. Each
instance may remain live as long the thread that stored it stays live.
Example:
```java
class C {
private final ThreadLocal<D> local = new ThreadLocal<D>();
public f() {
D d = local.get();
if (d == null) {
d = new D(this);
local.set(d);
}
d.doSomething();
}
}
```
The fix is often to make the field `static`:
```java
private static final ThreadLocal<D> local = new ThreadLocal<D>();
```
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