java - What is happens before relations in Synchronization? What is premature object leak? -

i reading synchronizedmethodsoracledoc , unable understand concept. says that.

warning: when constructing object shared between threads, careful reference object not "leak" prematurely. example, suppose want maintain list called instances containing every instance of class. might tempted add following line constructor: instances.add(this); other threads can use instances access object before construction of object complete.

what mean?

it states :-

second, when synchronized method exits,it automatically establishes happens-before relationship subsequent invocation of synchronized method same object.

what meaning of happens before relationship?

can elaborate on these 2 points? thanks.

premature leak

leaking reference object not yet created.

example:

class someclass{     public someclass(list list){         list.add(this); //this leaked outside before complete creation, constructor still not complete          //do other chores create object    } } 

some thread 2:

listpassedtosomeclass //thread using list , working on 

now on calling add on list constructor published this reference in list shared other thread, , can see reference added in list before constructor has ended (which means object not in stable state , not created).

there possibility thread 2 sorts of weird behavior on using object state unstable. avoid leaking references.

happens before