Four types of references in Java (strong, soft, weak, and virtual references to objects)

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.

⑴ Strong references (strongreference)
Strong references are the most commonly used references. If an object has a strong reference, the garbage collector will never recycle it.when there is not enough memory space,Java virtual machines would rather throw OutOfMemoryErrorerrors, causes the program to terminate abnormally, and does not rely on the arbitrary collection of strongly referenced objects to resolve out-of-memory issues。 PS: Strong references are actually the meaning of our usual a = new A ().

⑵ Soft Reference (SoftReference)
if an object has only soft references, enough memory 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).
A 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 references (weakreference)
The difference between a weak reference and a soft reference is:objects that have only weak references have a shorter life cycle. In the process of scanning the area of memory that the garbage collector thread governs, once an object with only a weak reference is found, it will reclaim its memory, 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.
A 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 adds the weak reference to the reference queue associated with it.

⑷ Virtual Reference (phantomreference)
"Virtual Reference" as the name implies, is the form of a dummy, and several other references are different,a virtual reference does not determine the life cycle of an object. If an object holds only virtual references, it can be recycled at any time by the garbage collector, just as there are no references。
Virtual references are primarily used to track activities that objects are 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 = new phantomreference (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.

to build 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 a soft reference is used
      SoftReference is characterized by an instance of it holding a soft reference to a Java object, 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, this 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 reclaiming these objects, we can pass:
Myobject anotherref = (MyObject) asoftref . Get ();  

Re- Get a strong reference to the instance . After the collection, the call to get () method can only get null.

3 Use Referencequeue to clear SoftReference
        As a Java object that has lost the soft reference object. SoftReference objects have 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 you are creating a SoftReference object, use a Referencequeue object 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

}

Full code:

Referencequeue q = new Referencequeue ();//Gets data and caches object obj = new Object (); softreference sr = new SoftReference (obj, q);//Object obj = (object) Sr.get () at next use, if (obj = = null) {  //* re-fetch after soft reference is recycled C1/>obj = new Object ();} Clean up the remaining soft reference object after SoftReference ref = Null;while ((ref = Q.poll ()) = null) {  //cleanup work}

Questions:

So now the question is, if there is more than one reference type for an object, how do you judge the accessibility of it? In fact, the rules are as follows:

1. The accessibility of a single reference chain is determined by the weakest reference type;
2. The accessibility of multiple reference chains is determined by one of the strongest reference types;

We assume that the reference to ① and ③ in Figure 2 is a strong reference, ⑤ is a soft reference, ⑦ is a weak reference, and for object 5 according to these two principles, the path ①-⑤ takes the weakest reference ⑤, so the path reference to object 5 is a soft reference. Similarly, ③-⑦ is a weak reference. The strongest reference is taken between the two paths, and object 5 is a soft object (which is recycled when oom is about to occur).

Soft references, weak references, and virtual references are abstract class Java.lang. Ref. Reference , and most of the operations related to the reference queue and GC are implemented in the abstract class Reference.

reference:http://blog.csdn.net/u011936381/article/details/11709245

Http://www.cnblogs.com/fsjohnhuang/p/4268411.html

Four types of references in Java (strong, soft, weak, and virtual references to objects)