1. Memory and data area
The memory managed by JAVA is divided into different data areas, and these data areas have different life cycles for different purposes. Some areas will always exist after the process is started, and some will be created or destroyed with the thread.
Two, JVM has five important data areas
| name | Whether to share | OutOfMemoryError | StackOverflowError | Features | Remarks |
|---|---|---|---|---|---|
| Program counter | X | X | X | Bytecode line number indicator | If the local method is executed, the count value is empty |
| JAVA virtual machine stack | X | √ | √ | Used to store information such as local variable table, operand stack, dynamic link, method exit, etc. | The so-called java stack generally refers to the java virtual machine stack or local variable table |
| Native method stack | X | √ | √ | Serve the native method (native) | |
| Method area | √ | √ | X | Store type information, constants, static variables, and code cache after just-in-time compilation that have been loaded by the virtual machine | |
| heap | √ | √ | X | Store instantiation data | The main object of GC recycling |
3. Objects and space
There are two ways to allocate space for new objects:
| method | alias | Applicable scene | GC algorithm | GC method |
|---|---|---|---|---|
| Bump the pointer | Pointer collision | JAVA heap memory is absolutely regular | Compact compression | Serial / ParNew |
| Free list | Free list | JAVA heap memory canine teeth | Sweep | CMS |
Two ways of object access positioning
| interview method | JAVA heap | Method area |
|---|---|---|
| Handle access | Handle pool: pointer to object instance data + pointer to object type data instance pool: object instance data | Object type data |
| Direct pointer | Pointer to object type data + object instance data | Object type data |
Four, OutOfMemoryError abnormal location
| Overflow area | Abnormal characteristics | Possible Causes | solution |
|---|---|---|---|
| Java heap | Java heap space | Memory leak, memory overflow | leak: View the reference chain of the leaked object to the GC Roots, so as to locate the creation location of the object that cannot be recycled. overflow: Expand the heap |
| Virtual machine stack, local method stack | unable to create native thread | Too many threads | Reduce the number of threads or reduce the stack memory |
| Method area | PermGen | jdk version | Upgrade version to 1.8, go to permanent generation |