Object creation

5、 Object creation

---

5.1、 Object storage

After the object is created, it will be stored in the heap , And generate a memory address .

stay JVM Objects in are mainly stored in the heap in three parts ：

1. Object head ： Including the layout of heap objects 、 type 、GC The state of 、 Basic information such as synchronization status and identification hash
   • Mark Word： Object's own runtime data ( Hash code 、GC Generational age 、 Thread lock information, etc ). stay 32 position JVM The middle length is 32bit, stay 64 Bit JVM Medium is 64bit.
   • Type a pointer ：JVM Determine the instance of which class the object is created by the type pointer
   • The length of the array （ Array objects ）
2. The instance data ： Store the attribute value of the object
3. Alignment filling ： Align bytes , Populated data , Not necessary . Is to make the fields appear only in the same CPU In the cache line of

5.2、 Object creation process

1. Programmers write a class

2. .java The file is compiled as .class file ： First, through Javac Compiler will .java To JVM Loadable .class Bytecode file

   ① Lexical parser ： Separate words by spaces 、 The operator 、 Control symbols and other information , formation token information flow , Pass to the grammar parser .
   ② Syntax parser ： hold token Information flows according to Java Grammar rules assemble idiom tree .
   ③ semantic analyzer ： Check whether the keywords are used properly 、 Whether the type matches 、 Whether the scope is correct, etc .
   ④ Generated bytecode ： Convert the information of the previous steps into bytecode .

3. Class loading check ： The virtual machine encountered keywords new, First, check whether the parameter corresponding to this instruction can locate the symbolic reference of this class in the constant pool , And check whether the class represented by this symbolic reference is loaded 、 Parsed and initialized . If not, execute class loading .

4. Class loading ： Place the bytecode file in Java In the virtual machine , Load through class loader . There are three modes of execution , Explain to perform 、JIT Compile implementation 、JIT Compiler and interpreter mixed execution （ Main stream JVM Default execution mode ）. The advantage of mixed mode is that the interpreter interprets the execution at startup , Save compilation time .

5. Make sure that after the class is loaded , The virtual machine allocates memory space for objects , The distribution method is Pointer collision 、 Free list Two kinds of .

   • Pointer collision ： The memory of the heap is regular , The used memory is divided to one side , The free one is on the other side , Use a pointer in the middle as the demarcation indicator , When allocating, move the point to the idle direction by a corresponding distance
   • Free list ： Memory is not regular , Fragmented , The virtual machine needs to maintain a list to record which memory is available , Find enough space to allocate , And update the list record .

   Creating objects requires thread safety , There are usually two ways

   • CAS+ Failure to retry ： An implementation of optimistic locking , Suppose there is no conflict to create , Retry in case of conflict .
   • TLAB： For each thread in Eden To allocate a piece of memory , When allocating memory, first in TLAB Distribute , Object greater than TLAB When the remaining memory or memory is exhausted , Then use CAS.

6. Initialize zero value ： The virtual machine initializes the allocated memory space to zero , Ensure that the object instance field is Java Can be used without assigning initial value .

7. Set object header ： Object is an instance of which class 、 Find different metadata 、 Hash 、GC Age etc.

8. perform init Method ： Initialize an initial object according to the program design

9. Through the instant compiler JT Compile bytecode files into local machine code .

10. Java Programs are initially interpreted and executed through interpreters , When the virtual machine finds that a method or block of code is running very frequently , It will be regarded as ” Hot code ”, There are two ways to detect hotspot code, which are based on sampling and counter , In order to improve the execution efficiency of hotspot code , The virtual machine compiles them into local machine code , Optimize the code as much as possible , The back-end compiler that does this at run time is called just in time compiler .

11. You can also use static precompiler AOT Compile the program directly into binary code related to the target machine instruction set .