Object creation process and object layout

One 、 Object creation process

1、 Object to create the underlying source code

The assembly code here is jvm Recognizable bytecode

1. 0： Open up a space in the stacking area , Attach the default value of the corresponding type to each member variable （ Semi initialized state ）
2. 4： Modulation construction method , Assign real values to each member variable
3. 7： Bind the object to the reference of the stack

Be careful ： Instruction reordering may occur during the execution of the above assembly code （ Disorderly execution ）. If it is executed in the final lazy mode of multithreading , This is likely to happen ： The first thread obtains the lock , Go to new Object but 4 and 7 Disorderly execution , That is, the object has no real assignment , Then the second thread comes , It is found that the object pointed to by the reference is not empty , Then take this object directly and leave , There's a problem . So it must be added volatile, To prevent disorder in the process of creating objects . This is also DCL（ The sluggard model ） Must be added volatile Why . The code is as follows ：