JVM understanding

Location ？

On top of the operating system .

Three jvm？

SUN The company's HotSpot- most

BEA The company's JRockit

IBM The company's J9 VM

Where to tune ？-heap

99% Reactor tuning memory . The stack is 100% There will be no rubbish .

Class loader ？

load class file , An object has only one Class,Class It's a template .

1. The loader of the virtual machine

2. Start class loader -rt.jar Internal

3. Extend the classloader -\jre\lib\ext Under the

4. Application Loader -classpath: Under the , Generally, the business written by programmers is loaded here

 null: Can't adjust , It's because of the use of C C++ Written

java = C++--,java Just get rid of C Tedious knitting and memory management .

Parent delegate mechanism ？ Security

The class loader receives a request to load , Delegate the request to the parent loader to complete , All the way up , Until you start the class loader Boot loader , The startup class cannot be loaded in the reverse direction , Until the load is complete , If the loading is not completed, an error will be reported Class Not Found.

and string Same name and same package , Define this class to execute the error report .

When it comes to execution, look for App-Ext-Boot, In the end in Boot An error is reported due to the execution under , Will give priority to Boot To load the , This will execute java.lang Under the String The method of class cannot be found main Method .

When in Boot I will go again when I can't find it Ext If you can't find it, go again App Look under .

Sandbox security mechanism sandbox? Understanding can

bytecode verifier ： Make sure java Class documents comply with java language norm . Not all files go through bytecode validators, such as core classes （java Own classes such as string Because it has been verified ）.

Class loader ：

      Parent delegate mechanism .

      Put the code in a protected domain , Determine what the code can do .

native？

Expand java The use of the class , Integrating different programming languages for my use . At first, I wanted to integrate C C++

A special local method stack is opened in memory Native Method Stack, Store local methods

Create a thread, Click in and you'll see start0() Method ,start0() The way is native Method .

By native Decorated methods will call the underlying C, Native Method Stack -> Local interface (JNI)-> Local method library .

Method area ？

Shared by all threads , All information defining the method is saved in this area , It belongs to the shared interval .

Stack ？stack

First in, then out , Last in, first out . bucket .

Each thread creates a stack , When the thread ends, the stack ends , The data in each stack is private .

Stack does not participate in garbage collection , The life cycle is short , Switch through stack frame .

Store the object ： local variable + References to objects + Example method . That is, things inside the method and method references .

queue ？FIFO

fifo . The Conduit .

main Method executes first and ends last ？

Pressing stack ,main Go first and start to adjust other business methods .

StackOverflowError Stack overflow ？

Recursive detuning method , Many methods are pushed into the stack .

Pile up -heap？

One jvm only one heap,stack It is thread level and can have multiple .

The heap size can be adjusted .

• The younger generation ： Class birth death zone .
• Old age ： Transitional area .
• Meta space ： The undead . memory-resident . Logically there is , There is no Physics .

Heap memory tuning ：-Xms1024m -Xmx1024m -XX:+PrintGCDetails

Pile up 、 Stack 、 Method area comparison ？

Pile up ：

Provide all class instances and array object storage areas .

jvm There's only one heap area (heap) Shared by all threads , Basic types and object references are not stored in the heap , Just store the object itself .

The stack area :

Each thread contains a stack area , The stack only stores references to objects of basic data type and custom objects ( Not object ), Objects are stored in the heap .

The data in each stack ( Original type and object reference ) It's all private , Other stacks cannot be accessed .

Method area :

It's also called static area , It's like a pile , Shared by all threads . The method area contains all the class and static Variable .

The method area contains elements that are always unique throughout the program , Such as class,static Variable .

The runtime constant pools are allocated in Java In the method area of virtual machine .