Java Strong reference Soft reference weak reference virtual reference

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Java four types of references include strong references, soft references, weak references, and virtual references.

Strong references:

The garbage collector will never recycle as long as the reference exists
Object obj = new Object ();
The corresponding object, such as Obj.equels (new object), can be obtained directly from obj.
And so the Obj object is a strong reference to the later new object, and the object will be freed only if obj is released, which is also the encoding that we often use.

Soft references:

Non-mandatory references, which are recycled before memory overflows, can be implemented with the following code
Object obj = new Object ();
softreference<object> SF = new softreference<object> (obj);
obj = null;
Sf.get ();//sometimes returns null
In this case, SF is a soft reference to obj, which can be retrieved through the Sf.get () method, which, of course, returns NULL when the object is marked as an object that needs to be recycled;
Soft reference the main user to implement a similar caching function, in memory enough to directly through the soft reference value, no need to query data from the busy real source, improve speed, when the memory is low, automatically delete this part of the cached data, from the real source to query the data.

Weak references:

The second garbage collection is recycled and can be implemented with the following code
Object obj = new Object ();
weakreference<object> wf = new weakreference<object> (obj);
obj = null;
Wf.get ();//sometimes returns null
Wf.isenqueued ();//Returns whether the garbage collector is marked as garbage that is about to be recycled
The weak reference is reclaimed at the second garbage collection, the corresponding data is taken from the weak reference in a short time, and null is returned when a second garbage collection is performed.
Weak references are primarily used to monitor whether objects have been flagged by the garbage collector as garbage that is about to be reclaimed, and can be returned by a weakly referenced isenqueued method to whether the object is flagged by the garbage collector.

Virtual References:

Garbage collection is recycled and cannot be referenced to object values, and can be implemented with the following code
Object obj = new Object ();
Phantomreference<object> PF = new phantomreference<object> (obj);
Obj=null;
Pf.get ();//always returns null
Pf.isenqueued ();//Returns whether it has been removed from memory
Virtual references are reclaimed every time garbage collection is made, and the data that is obtained by a virtual reference to the Get method is null and therefore is also a ghost reference.
Virtual references are primarily used to detect whether an object has been removed from memory.

Recently in the study of Java Virtual machine, encountered the problem of reference, in this reference to summarize:

Original address: http://blog.csdn.net/coding_or_coded/article/details/6603549

Strong, soft, weak, and virtual references to objects
In previous versions of JDK 1.2, if an object was not referenced by any variable, the program could no longer use the object. That is, only the object is in the accessible (reachable) state before the program can use it. Starting with JDK version 1.2, the reference to the object is divided into 4 levels, giving the program more flexibility in controlling the object's life cycle. These 4 levels are high to low in order: Strong references, soft references, weak references, and virtual references.

The

⑴ Strong reference (strongreference)
Strong reference is the most commonly used reference. If an object has a strong reference, the garbage collector will never recycle it. When there is not enough memory space, the Java virtual Machine prefers to throw a outofmemoryerror error, which causes the program to terminate abnormally, and does not rely on random recycling of strongly referenced objects to resolve out-of-memory issues.   ps: A strong quote is actually what we usually mean by a A = new A ().

⑵ Soft Reference (softreference)
If an object has only soft references, enough memory space is available, the garbage collector does not recycle it, and if the memory space is insufficient, the memory of those objects is reclaimed. The object can be used by the program as long as it is not reclaimed by the garbage collector. Soft references can be used to implement memory-sensitive caches (see below for an example). The
Soft reference can be used in conjunction with a reference queue (Referencequeue), and if the object referenced by the soft reference is reclaimed by the garbage collector, the Java Virtual machine will add the soft reference to the reference queue associated with it.

⑶ Weak reference (WeakReference)
The difference between a weak reference and a soft reference is that an object with only a weak reference has a more ephemeral life cycle. As the garbage collector thread scans the area of memory it governs, once an object with only a weak reference is found, its memory is reclaimed, regardless of whether the current memory space is sufficient or not. However, because the garbage collector is a low-priority thread, it is not necessarily quick to discover objects that have only weak references. The
Weak reference can be used in conjunction with a reference queue (Referencequeue), and if the object referenced by the weak reference is garbage collected, the Java Virtual machine will add the weak reference to the reference queue associated with it.

⑷ Virtual Reference (phantomreference)
"virtual reference" as the name implies, is a dummy, unlike several other references, a virtual reference does not determine the object's life cycle. If an object holds only virtual references, it can be reclaimed by the garbage collector at any time, just as there are no references. The
Virtual reference is primarily used to track the activity that an object is reclaimed by the garbage collector. One difference between a virtual reference and a soft reference and a weak reference is that the virtual reference must be used in conjunction with the reference queue (Referencequeue). When the garbage collector prepares to reclaim an object, if it finds that it has a virtual reference, it will add the virtual reference to the reference queue associated with it before reclaiming the object's memory.

Referencequeue queue = new referencequeue ();

Phantomreference PR =NewPhantomreference (object, queue);

The program can see if the referenced object is going to be garbage collected by judging whether the reference queue has been added to the virtual reference. If the program discovers that a virtual reference has been added to the reference queue, it can take the necessary action before the memory of the referenced object is reclaimed.
building a cache of sensitive data using soft references
1 Why you need to use soft references

First, we look at an example of an employee information query system. We will use an employee information query system implemented in the Java language to query employee profile information stored in a disk file or database. As a user, we may have to go back and look at employee profile information that was viewed a few minutes or even seconds ago (again, we often use the back button when we browse the Web page). At this point we usually have two ways of implementing the program: one is to keep the employee information viewed in the past in memory, the life cycle of each Java object that stores the employee profile information throughout the application, and the other is when the user starts to view the profile information of the other employee, Ends a reference to the Java object that stores the currently viewed employee profile information, allowing the garbage collection thread to reclaim the memory space it occupies, and reconstruct the employee's information when the user needs to browse the employee's profile information again. Obviously, the first implementation will result in a lot of memory waste, and the second implementation is flawed even if the garbage collection thread has not been garbage collected, the object containing the employee profile information is still intact in memory, and the application is rebuilding an object. We know that access to disk files, access to network resources, query databases and other operations are important factors affecting the performance of application execution, if you can regain those who have not yet been recycled Java object references, will reduce unnecessary access, greatly improve the speed of the program.

2 If you use a soft reference
The SoftReference feature is that an instance of it holds a soft reference to a Java object, and the existence of the soft reference does not prevent the garbage collection thread from reclaiming the Java object. That is, once SoftReference has saved a soft reference to a Java object, the Get () method provided by the SoftReference class returns a strong reference to the Java object before the garbage thread recycles the Java object. Also, once a garbage thread reclaims the Java object, the Get () method returns NULL.
Look at the following code:

MyObject aRef = new
MyObject ();

SoftReference asoftref=New softreference (AREF);
At this point, for this MyObject object, there are two reference paths, one is a soft reference from the SoftReference object, and one is a strong reference from the variable areference, so this MyObject object is a strong object.
We can then end Areference's strong reference to this MyObject instance:

AREF = null;

Since then, the MyObject object has become a soft and accessible object. If the garbage collection thread is in memory garbage collection, it does not always retain the object because it has a softreference reference to the object. The Java Virtual machine's garbage collection thread treats soft-and object-and other generic Java objects differently: The cleanup of soft and object is determined by the garbage collection thread according to its memory requirements based on its particular algorithm. That is, the garbage collection thread reclaims the soft objects before the virtual machine throws OutOfMemoryError, and the virtual machine takes priority to reclaim the soft objects that are unused for long periods of time, and the "new" soft-anti-objects that have just been built or used are retained by the VM as much as possible. Before we recycle these objects, we can pass:
MyObject anotherref= (MyObject) asoftref.get ();
Regain a strong reference to the instance. After the collection, the call to get () method can only get null.

3 Use Referencequeue to clear the softreference that lost the soft reference object
As a Java object, the SoftReference object has the generality of Java objects in addition to the particularity of saving soft references. So, when a soft and an object is recycled, although the get () method of the SoftReference object returns NULL, the SoftReference object no longer has the value of being present and requires an appropriate cleanup mechanism. Avoid memory leaks caused by a large number of SoftReference objects. The Referencequeue is also available in the Java.lang.ref package. If a SoftReference object is created, a Referencequeue object is used as the constructor for the softreference, such as:

Referencequeue queue = new
Referencequeue ();

SoftReference
ref=New
SoftReference (amyobject, queue);
Then when the SoftReference soft-referenced amyohject is reclaimed by the garbage collector, ref's strongly referenced SoftReference object is included in the Referencequeue. That is, the object saved in Referencequeue is the reference object, and it is the reference object that has lost the object it is soft referenced. Also from the name of Referencequeue, it is a queue, when we call its poll () method, if this queue is not empty queue, then will return the queue in front of the reference object.
At any time, we can call Referencequeue's poll () method to check if there are non-strong objects that it cares about being recycled. If the queue is empty, a null is returned, otherwise the method returns a reference object in front of the queue. Using this method, we can check which softreference the soft reference object has been recycled. So we can get rid of the SoftReference objects that have lost the soft reference object. The usual ways are:

SoftReference ref = NULL;

while (ref = (EMPLOYEEREF) q.poll ())! = null) {

Clear ref

}

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Java Strong reference Soft reference weak reference virtual reference