JVM GC garbage collection brief

GC

in the light of java Pile recycling .

Reachability analysis algorithm ： from gc root Search for reference chains .

Eden Trigger when area is full minor gc, The surviving object enters survivor District （ Copy algorithm ）, Age plus 1, When the age of the object is greater than the threshold （ Default 15） when , Into the old age . You can set the threshold through parameters , The virtual machine will be dynamically adjusted during operation . When a certain age exceeds survivor Half of the area （ Default 50%） when , Take this age and MaxTenuringThreshold As the new threshold .

The new generation （ Mark - Copy algorithm ）

• Mark living objects - Because there are fewer surviving objects
• Memory size eden：to：from = 8 : 1 : 1
• minor gc after ,Eden The objects that live in the area are copied to “survivor To”;“survivor From” The living objects in the zone , When the age reaches the threshold, we enter the old generation , Otherwise it will be copied to “survivor To” in .survivor From and survivor To swap .
• survivor From Out of space , The part that cannot be put down will enter the elderly generation ahead of time .

Old age （ Mark - eliminate - Sorting algorithm ）

When there is not enough memory in the old age full gc（ Mark - eliminate ）, many times full gc After finishing . Frequency parameter CMSFullGCsBeforeCompaction The default is 0 Time .

Strong citation ：gc No recovery .
Soft citation ： Only recycle when memory is insufficient .
Weak reference ：gc Recycling .
Virtual reference ： Before recycling , Get the system notification , Some treatment can be done before recycling .

java Memory model

1. Program counter ： Control program execution .
2. Java Virtual machine stack ： Store local variable table 、 The stack of operands 、 Dynamic connection 、 Method export, etc .
3. Native Method Stack ： Store local （native） Methods information .
4. Java Pile up ： Stored object instance .
5. Method area ： Storing static data （ Constant 、 Static variables, etc ）. Contains the runtime constant pool ： Store constants .

Java Heap reclamation Algorithm

1. Reference counting （ Most virtual machines do not ）
2. Reachability analysis algorithm
   according to “ Root object ” Search its “ References to the chain ” Judge whether an object is reachable , Filter unreachable objects , If the object exists finalize() Method , also finalize() Method has not been implemented , Then put the object in the queue , from Finalizer The thread executes its finalize() Method to release the object . If the object in the queue is executing finalize() Re referenced before release , Will be moved out of the release queue .JDK9 After being removed .

Recovery object judgment

1. Reclaim string constants that do not have references （ String constant pool in heap , The runtime constant pool is in the method area ）
2. Class not in use , just “ Allowed to recycle ”
   At the same time satisfy ：
   All instances of this class are recycled ;
   The class loader for this class has been recycled ;
   Of the class Class The object does not exist and is referenced , Unable to launch access .

GC The trigger condition

Minor GC The trigger condition ：Eden Area is full .

Full GC The trigger condition ：

1. call System.gc when , System recommendation implementation Full GC, But not necessarily .
2. There is not enough space in the old age .
3. Insufficient method space .
4. adopt Minor GC The average size of backward into old age is larger than the available memory of old age .
5. from Eden District 、From Space District direction To Space When copying , The object size is greater than To Space Available memory , Then transfer the object to the old age , And the available memory of the old age is smaller than the size of the object .

【 Add 】 JDK 7 Before ,HotSpot When using the permanent generation to implement the method area , Implementation is completely in line with this logical concept . And in the JDK 7 And after ,HotSpot Already put the string constant pool originally in the permanent generation 、 Static variables, etc. are moved to the heap , At this time, class variables will follow Class Objects are stored together in Java In the pile .