Strong references, soft references, weak references, virtual references
Let's talk about the garbage collection mechanism gabagecollection:
GC is the meaning of garbage collection (gabagecollection). Like Java, the Android system is also powered by GC to reclaim memory. Android app launch, Android system will allocate a DALIVK virtual machine for this application, so that the application runs on this standalone virtual machine, but a virtual machine memory size is generally limited (20M or so). It can be seen that an application that runs the requested memory may not be infinitely large.
Typically, the GC is run in a separate, relatively low-priority thread. This frees the memory to be processed by the GC and the GC will monitor and release "invalid" Memory in real time. The keyword "Invalid" Here causes our attention, what memory unit is invalid? The general GC approach is to use a reference counting method to determine whether an internal storage element (a variable) is invalid memory, in fact, many systems have adopted this approach, such as cocos2d the game developed by the engine also adopted this reference counting method. The reference counting method is simply referred to as a variable or a piece of memory is currently referred to the number of times, if the number of references equals 0, it can be said that the variable or the memory is not referenced, then the GC "very likely" will release him. Here's to mention: "Very likely"? I think this is closest to real-time, first the GC is running on a lower priority thread, and then the specific work of GC recycling is quite complicated (note that the reference counting method is only a common method in GC, and the GC has a complex approach to memory management, such as the use of chronological methods, can go to research), such as the need to release a lot of memory at the same time, and CPU resources are relatively tense, GC may selectively release some memory resources. When we do Android development, we need to pay special attention to memory references, such as a memory-intensive object, and a bitmap object, they are often useless, but also occupy a lot of memory resources and GC has no way to release, resulting in serious consequences. The general principle is: to occupy a large amount of memory variables, after the use of the active set to NULL, if possible to actively call GC:SYSTEM.GC (), especially some static type of reference risk is very large.
understand the GC as well as the reference counting method, here are a few references :
Concept Introduction
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. Apply the class hierarchy to the object.
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, 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.
Soft References (softreference)
if an object has only soft references, theThe garbage collector will not recycle it if there is enough memory space, and the memory of these objects will be reclaimed if there is not enough memory space.. 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.
Weak references (weakreference)
the difference between a weak reference and a soft reference is that an object with only a weak reference has a shorter life cycle. While the garbage collector thread scans the area of memory it governs,once an object with only a weak reference is found, it will reclaim its memory, regardless of the current memory space. 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.
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.
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:virtual references 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.
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 lifecycle of every Java object that stores employee profile information throughout the application is always the same;The other is when the user starts to view the other employee's profile information, the end reference of the Java object that stores the currently viewed employee profile information, so that the garbage collection thread can reclaim its occupied memory space .when the user needs to browse the employee's profile information again, rebuild the employee's information. 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.
Suppose our app uses a lot of default images, such as the default avatar in the app, the default game icon, and so on, which can be used in many places. If you read the picture every time, the slow speed of reading the file requires hardware operation, which results in lower performance. So we're thinking about caching the images and reading them directly from memory when needed. However, because the picture occupies a large memory space, caching many images requires a lot of memory, it is likely to be more prone to outofmemory exceptions. In this case, we can consider using soft-referencing techniques to avoid this problem.
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.
use Referencequeue to clear softreference that have lost soft reference objects
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
}
after understanding Referencequeue's working mechanism, we can begin to construct a cache of Java objects.
here is just a collection of some concept introduction, because the time problem I did not specifically hit the code practice, this later I think I will go to attach their source code. Give yourself an encouragement and cheer up. Quickly get rid of rookie status.
The original address of the surface reference http://www.apkbus.com/forum.php?mod=viewthread&tid=13534&extra=&page=1
Reference Address http://www.jb51.net/article/36627.htm
Strong references, soft references, weak references, virtual references
