【 interview ： Concurrent articles 14： Multithreading ： Monitor Concept 】

00. Preface

If you have any questions, please point out , thank .

01.Java Object head

With 32 For example, bit virtual machine

Common object

explain

The object contains a header ：Mark Word（32bits）Klass Word（32bits）

Klass Word（32bits）： It's a pointer It points to the class it belongs to

Mark Word（32bits）： Here's the explanation

Array objects

There is one more array object array length（32bits） Used to specify the length of the array

Mark Word structure

02.Monitor

Introduce

Monitor Translated as monitor or tube side , every last java Objects can be associated with a Monitor object , If you use synchronized Lock the object （ Heavyweight lock ） after , Change object header MarkWord Is set to point to Monitor Object pointer

Picture explanation

We can see that this time is Two threads have not yet executed Critical area code block , So at this time obj Object's MarkWord For the above MarkWord In state normal state .

after Thread-1 First, the critical area code is executed , So at this time obj Object's MarkWord The state of locking in the middle becomes 10 The state becomes Heavyweight Locked, Now? obj The head of the object MarkWord Is set to point to Monitor The pointer to , And at this point Monitor Under surveillance Thread-1.

then Thread-2 To get the lock Found to have been occupied , Then it is placed in the blocking list EntryList.