Java Learning Note 10

When a program creates a reference-type entity such as an object, an array, and so on, the system allocates a chunk of memory in the heap memory, and the object is stored in the memory area.

When the object that we create is no longer referenced, the memory becomes garbage, and finally waits for the garbage collection mechanism to be recycled, and the Java garbage collector

System has the following characteristics:

1, garbage collection mechanism is only responsible for recovering objects in the heap memory, does not reclaim any physical resources, such as database connections, network IO and other resources.

2, the program can not accurately control the operation of garbage collection, garbage collection will be at the appropriate time, when the object permanently lost the reference, the system will be in

Reclaim the memory it occupies when it is appropriate.

3. Before the garbage collection mechanism reclaims any objects, it will always call its finalize () method, which allows a reference variable to refer to the object again, from

This causes the garbage collection mechanism to cancel the recycle.

When an object is running in heap memory, it can be divided into the following three states, depending on the state it refers to in the referenced variable:

1, up to the state: When an object is created, if more than one reference variable refers to it, then the object in the program can be reached, the program can

Call the field and method of the object by referencing the variable.

2. Recoverable state: If an object in the program no longer has any reference variable referencing it, it enters the recoverable state, in which case the system

Garbage collection mechanism is ready to reclaim the memory that the object occupies, and the system invokes the Finalize () method of all recoverable state objects before the object is reclaimed

for resource cleanup. If the system re-enables a reference variable to refer to the object when it calls the Finalize () method, the object will again become reachable

Otherwise, the object will enter the unreachable state.

3. Unreachable state: When an object's association with all reference variables is severed, and the system has called the Finalize () method of all objects, it still does not make the

When an object becomes unreachable, the object will permanently lose its reference and eventually become unreachable, and when an object is in an unreachable state, the system

The system will actually reclaim the memory that the object occupies.

For most objects, there is a reference variable referencing the object in the program, and the most common way to refer to it is in addition to the JAVA.LANG.REF package.

3 classes are available: SoftReference, phantomreference, and WeakReference, respectively, representing the system's 3 references to the object

Type: Soft references, virtual references, and weak references, Java references to objects are in the following 4 ways:

1, strong reference: This is the most common method of reference in Java programs, the program creates an object, and assigns the object to a reference variable, the program passes

The reference variable to manipulate the actual object. When an object is referenced by one or more reference variables, it is in a state of reach and cannot be tied

garbage collection mechanism.

2, soft reference: Soft references need to be implemented through the SoftReference class, when an object has only soft references, it may be garbage collection mechanism back

For a soft reference object, when the system memory space is sufficient, it will not be reclaimed by the system, the program can also use the object, when the system memory

When there is not enough space, the system may reclaim it. Soft references are typically used in memory-sensitive programs.

3. Weak reference: Weak citation is implemented through the WeakReference class, weak references and soft references are similar, but weak references have lower reference levels, for only weak

Referenced object, but the system garbage collection mechanism runs, regardless of whether the system memory is sufficient, will always reclaim the memory occupied by the object. Of course, and

This is not to say that when an object has only a weak reference, it is immediately recycled.

4. Virtual reference: The virtual citation is implemented by the Phantomreference class, and the virtual reference is exactly similar to no reference. Virtual references do not have too much impact on the object itself

The object does not even feel the existence of a virtual reference. If an object has only one virtual reference, it is roughly the same as a non-referenced effect. Virtual Primer

The virtual reference cannot be used alone, and the virtual reference must be combined with the reference queue (Referencequeue) in order to be used primarily to track the state of the object being garbage collected

Use.

The reference object is represented by the Java.lang.ref.ReferenceQueue class, which is used to hold a reference to the object that was recycled when the union uses soft references, weak references, and

When a queue is referenced, the system adds a reference to the associated reference queue after the referenced object is reclaimed. With soft references and weak

By contrast, a virtual reference adds a virtual reference to its object to its associated reference queue before the object is disposed, which allows the object to be

Take action before recycling.

Soft and weak references can be used alone, but virtual references cannot be used alone, and using virtual references alone does not make much sense, and the primary role of virtual references is

The tracking object is garbage collected, and the program can understand if the virtual reference is already included in the reference queue associated with the virtual reference

Whether the referenced object is about to be reclaimed.

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Java Learning Note 10