day6-jvm

Today's summary

jvm Dark horse 108-139p

JVM

2 Bytecode instruction

javap -v xxx.class

Bytecode execution process

main Method class loading , Put it in the method area , Constant pool （class data ） Into the runtime constant pool
Small numbers are put in the bytecode instructions of the put method , Put it in the constant pool beyond the range

• x=x++ Why is it useless

Because first x Put it on the operand stack , then x++, Finally, the value of the operand stack is given to x, At this time, it's still 0.
i++ First load Since the increase again ,++i It is to increase first and then load

• cinit

The compiler is in top-down order , Code that assigns values to all static code blocks and static members , Merge into a special method ,()V

• init

Instance object .
init Method , Also in order , The construction method finally executes

• Method call

Construction method ：invokespecial
private ：invokespecial
final：invokespecial
public：invokevirtual（ Because it is possible to rewrite ）
Interface method ：invokeInterface( Support polymorphism ）
Static methods ：invokestatic
new Method ： Allocate memory to objects in the heap — After successful allocation, put the reference of the object into the operand stack — Make a copy of the elements at the top of the stack —invokespecial Execute the construction method of the top element of the stack and remove — Save the remaining one out of the stack into the local variable table
Do not call static methods through objects 、 Variable , Two useless virtual machine instructions will be generated

• The principle of polymorphism

The virtual method table will be generated in the link phase , The entry address of the virtual method table is generated

1. Find the object through the object reference of stack frame China 、
2. Analysis object header , Find the object's actual classsss
3. class In the structure vtable, In the class loading phase, it has been generated according to the method rewrite rules
4. Look up the table to get the specific address of the method
5. The bytecode of the execution method

• Exception handling

There is one Exception table, monitoring try Code , Type matching after exception , See whether it is consistent or subclass exception .
Multiple catch： There are multiple in the exception table . Because there will only be one exception , So the local variable table only occupies one slot （ Save the memory of stack frame ）
multi-catch: There are multiple exceptions .xxx E | xxxE | xxxE: Three different types of exception entries are the same .

• finally

take finally The code of is put in try The piece is put again catch block
except try and catch Which cannot be captured in , Will jump to the end , The type is any, To carry out , Of course, it is also copied finally Things that are

20.
try There is... In it finally Code for , Finally, it must be implemented .
So don't be finally Write in return, Because it may be swallowed abnormally

10
No, return sentence , There will be one. slot To save , And it will throw abnormal

• synchronized

How to lock and unlock in pairs ？
First a_load hold lock Object is loaded into the operand stack , Express synchronized Start
Then copy the reference and put it in the slot , to monitorenter and monitorexit To use （ Two will consume lock quote ）
monitorenter Start lock quote
Before loading lock, perform monitorexit Unlock , The same goes for exceptions , Will load lock References to objects , To unlock .

3 Compile time processing

Optimization and processing during compilation ： Grammatical sugar （ The compilation process automatically generates and converts some code ）

1. Default constructor

The default constructor will be generated （ Without writing it yourself ）, Because everything is inherited from Object.

2. Automatic disassembly box

int->Integer： Packing
Integer->int： Unpacking

3. Generic set values

Normal erasure ： After compilation, it is uniformly set to Object
When you return, you return Object： use checkcast, take object become Integer（ It is equivalent to automatically doing a forced type conversion ）
The information in the method body is erased , The paradigm of local variables has not been erased , But you can't get it by reflection , Only the parameters and return values of the method can be obtained through reflection

4. Variable parameters

String …args -> String[] args
If parameters are not passed , The transmission is an empty String Array , instead of null value

5. foreach

Array assignment can be directly int arr[] = {}, Will become int arr[] = new int[]{}
Array foreach It's actually becoming fori
list Of foreach Is the iterator used （ Realized Iterator All of them can be used ）

6. switch character string

Actually, it's more hash code

• Why is it more hashcode, Use again equals Comparison? ？

hashcode To improve efficiency , Reduce comparison ,equals To compare hash Conflict ,
Because most of them hashcode It's all unique .

• Why rewrite hashcode and equals？
  • Used when only one value needs to be compared , For example, different objects ,id The same can be regarded as the same

equals equal ,hashcode It must be equal ,hashcode For quick comparison ,equals For absolute equality .hashcode There are conflicting situations , So further use equals Compare

• Why rewrite equals Must be given again i forehead hashcode
  • If two objects are equal , be hashcode ⼀ It must be the same
  • Two objects are equal , The two one. equals() ⽅ I can't go back to true
  • Two objects have the same hashcode value , They don't ⼀ Must be equal
  • Sum up , equals() ⽅ The law is covered , be hashCode() ⽅ Law must also be covered. This part is borrowed from ：https://juejin.cn/post/7025465769307668516,

7. switch enumeration

ordinal（）

8. enumeration

The essence is a class, Values are actually instance objects
It's a final Of cannot be inherited , Inherited the enumeration parent class （ Support the paradigm ）


Constructor private
$values Put the enumerated object into the array

It's resumed today

I had a long sleep in the morning , I played ball for a while in the afternoon , Still slack .
leetcode Do it , The code says , Project deployment .
I'll also look at the interview questions tomorrow .