Connor learns from JVM - the method of judging garbage collection

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Preface

Hey, Welcome to Connor learn JVM series , This series records my JVM Basic knowledge learning 、 The double check process , Welcome everyone to read axzheng ！ Originality is not easy. , Reprint please indicate the source ：http://t.csdn.cn/l2sBr, Without much to say, we'll start right away ！

Judge the method of garbage collection

1. Reference counting algorithm

Add a reference counter to the object

（1） Whenever there's a place to quote it , Counter value + 1

（2） When the reference fails , Counter value - 1

（3） The counter at any time is 0 It is impossible to use the object of

Under special circumstances, a lot of extra processing is required to ensure correct work , For example, a single reference count is difficult to solve the problem of mutual reference between objects

2. Reachability analysis algorithm

Through a series of GC Roots The root object serves as the starting node set , Start with these nodes , Search down... Based on reference relationships , The path the search process takes is called a chain of references , If an object arrives at GC Roots There is no chain of references between , From GC Roots Not reachable to this object , It is proved that this object can no longer be used

[ Failed to transfer the external chain picture , The origin station may have anti-theft chain mechanism , It is suggested to save the pictures and upload them directly (img-hpzoIiot-1658108100814)(F:\yhy925\dailylife\Work\1.Java\JVM\Resources\3. Garbage collector \1. Accessibility analysis .png)]

Accessibility analysis and finalize()

But such objects are still in “ Probation ” Stage

（1） After the accessibility analysis, it is found that the object is not related to GC Roots The linked chain of references , Then it will be marked for the first time

（2） Make a screening , Depending on whether the object overrides finalize() Method or finalize() Method has been called by the virtual machine , To determine whether it needs to be implemented finalize()

• No need to execute , That is, the object is not covered finalize() Method or finalize() Method has been called by the virtual machine
• You need to perform , Place the object in F-Queue In line , And later, a virtual machine automatically establishes 、 Low scheduling priority Finalizer Execute in thread finalize() Method . The execution here only refers to calling the method , There is no guarantee of waiting for it to finish , in case finalize() Slow execution leads to system crash

（3） The collector will respond to F-Queue The objects in are marked a second time , If the object is to be finalize() To save yourself —— Re associate with any object on the reference chain , Such as the this Assign to a class variable or a member variable of an object , Then it will be removed on the second mark “ About to be recycled ” Set , Otherwise, it will still be recycled

but finalize() High operation cost , High uncertainty , The order in which the individual objects are invoked cannot be guaranteed , It is not recommended to use

Can be used as GC Roots The object of

（1） The object referenced in the local variable table of the stack frame , Such as method parameters 、 Local variables, etc

（2） In the method area, the class static Property reference object , Such as reference type static Variable

（3） Objects referenced by constants in the method area , Such as the reference in the string constant pool

（4） In the local method stack JNI Referenced object

（5）JVM Internal references , For example, the basic data type corresponds to Class object 、 system class loader 、 Some resident exception objects (NPE、OOM)

（6） All objects held by synchronous locks