摘自Ethan Nicholas的《理解弱引用》:

Weak references A weak reference, simply put, is a reference that isn't strong enough to force an object to remain in memory. Weak references allow you to leverage the garbage collector's ability to determine reachability for you, so you don't have to do it yourself. You create a weak reference like this: WeakReference weakWidget = new WeakReference(widget); and then elsewhere in the code you can use weakWidget.get() to get the actual Widget object. Of course the weak reference isn't strong enough to prevent garbage collection, so you may find (if there are no strong references to the widget) that weakWidget.get() suddenly starts returning null. ... Soft references A soft reference is exactly like a weak reference, except that it is less eager to throw away the object to which it refers. An object which is only weakly reachable (the strongest references to it are WeakReferences) will be discarded at the next garbage collection cycle, but an object which is softly reachable will generally stick around for a while. SoftReferences aren't required to behave any differently than WeakReferences, but in practice softly reachable objects are generally retained as long as memory is in plentiful supply. This makes them an excellent foundation for a cache, such as the image cache described above, since you can let the garbage collector worry about both how reachable the objects are (a strongly reachable object will never be removed from the cache) and how badly it needs the memory they are consuming.

Peter Kessler补充道:

The Sun JRE does treat SoftReferences differently from WeakReferences. We attempt to hold on to object referenced by a SoftReference if there isn't pressure on the available memory. One detail: the policy for the "-client" and "-server" JRE's are different: the -client JRE tries to keep your footprint small by preferring to clear SoftReferences rather than expand the heap, whereas the -server JRE tries to keep your performance high by preferring to expand the heap (if possible) rather than clear SoftReferences. One size does not fit all.

应该注意，弱引用对象只有在只有弱引用时才会被收集。如果它有一个强引用，那么无论它有多少弱引用，它都不会被收集。

Java中的六种对象可达性状态:

Strongly reachable objects - GC will not collect (reclaim the memory occupied by) this kind of object. These are reachable via a root node or another strongly reachable object (i.e. via local variables, class variables, instance variables, etc.) Softly reachable objects - GC may attempt to collect this kind of object depending on memory contention. These are reachable from the root via one or more soft reference objects Weakly reachable objects - GC must collect this kind of object. These are reachable from the root via one or more weak reference objects Resurrect-able objects - GC is already in the process of collecting these objects. But they may go back to one of the states - Strong/Soft/Weak by the execution of some finalizer Phantomly reachable object - GC is already in the process of collecting these objects and has determined to not be resurrect-able by any finalizer (if it declares a finalize() method itself, then its finalizer will have been run). These are reachable from the root via one or more phantom reference objects Unreachable object - An object is neither strongly, softly, weakly, nor phantom reachable, and is not resurrectable. These objects are ready for reclamation

欲了解更多详细信息:https://www.artima.com/insidejvm/ed2/gc16.html«崩溃

Weak references are collected eagerly. If GC finds that an object is weakly reachable (reachable only through weak references), it'll clear the weak references to that object immediately. As such, they're good for keeping a reference to an object for which your program also keeps (strongly referenced) "associated information" somewere, like cached reflection information about a class, or a wrapper for an object, etc. Anything that makes no sense to keep after the object it is associated with is GC-ed. When the weak reference gets cleared, it gets enqueued in a reference queue that your code polls somewhere, and it discards the associated objects as well. That is, you keep extra information about an object, but that information is not needed once the object it refers to goes away. Actually, in certain situations you can even subclass WeakReference and keep the associated extra information about the object in the fields of the WeakReference subclass. Another typical use of WeakReference is in conjunction with Maps for keeping canonical instances.

另一方面，softreference对于缓存外部的、可重新创建的资源很有好处 因为GC通常会延迟清除它们。尽管如此，这是有保证的 在抛出OutOfMemoryError之前，sofreferferences将被清除 理论上不会引起OOME[*]。

Typical use case example is keeping a parsed form of a contents from a file. You'd implement a system where you'd load a file, parse it, and keep a SoftReference to the root object of the parsed representation. Next time you need the file, you'll try to retrieve it through the SoftReference. If you can retrieve it, you spared yourself another load/parse, and if the GC cleared it in the meantime, you reload it. That way, you utilize free memory for performance optimization, but don't risk an OOME.

现在是[*]。保持一个SoftReference本身不会导致OOME。如果 另一方面，你错误地把软参考用在了一个弱参考的任务上 (也就是说，你以某种方式保存与对象相关的信息 ，并在Reference对象获得时丢弃它 清除)，您可以运行OOME作为您的代码轮询referencqueuue 而丢弃关联对象可能会发生不及时运行的情况 时尚。

所以，决定取决于使用情况 -如果你缓存的信息构造起来很昂贵，但是 尽管如此，可从其他数据重构，使用软引用 -如果您保留对某些数据的规范实例的引用，或者 您希望拥有对对象的引用，但不“拥有”它(因此 防止它被GC)，使用弱引用。

弱引用 http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/ref/WeakReference.html

原理:弱引用与垃圾回收相关。通常，具有一个或多个引用的对象将不符合垃圾收集的条件。 上述原则在弱参考时不适用。如果一个对象对其他对象只有弱引用，那么它就可以进行垃圾收集了。

让我们看看下面的例子:我们有一个Map with Objects，其中Key是引用一个对象。

import java.util.HashMap;   
public class Test {

    public static void main(String args[]) {
        HashMap<Employee, EmployeeVal> aMap = new 
                       HashMap<Employee, EmployeeVal>();

        Employee emp = new Employee("Vinoth");
        EmployeeVal val = new EmployeeVal("Programmer");

        aMap.put(emp, val);

        emp = null;

        System.gc();
        System.out.println("Size of Map" + aMap.size());

    }
}

现在，在程序执行期间，我们使emp = null。持有键的Map在这里没有意义，因为它是空的。在上述情况下，对象不会被垃圾收集。

WeakHashMap

在WeakHashMap中，当不再可能从Map中检索条目时，条目(键到值的映射)将被删除。

让我用WeakHashMap展示上面的例子

import java.util.WeakHashMap;

public class Test {

    public static void main(String args[]) {
        WeakHashMap<Employee, EmployeeVal> aMap = 
                    new WeakHashMap<Employee, EmployeeVal>();

        Employee emp = new Employee("Vinoth");
        EmployeeVal val = new EmployeeVal("Programmer");

        aMap.put(emp, val);

        emp = null;

        System.gc();
        int count = 0;
        while (0 != aMap.size()) {
            ++count;
            System.gc();
        }
        System.out.println("Took " + count
                + " calls to System.gc() to result in weakHashMap size of : "
                + aMap.size());
    }
}

输出:对System.gc()进行了20次调用，导致高德地图大小为:0。

WeakHashMap只有对键的弱引用，没有像其他Map类那样的强引用。虽然使用了WeakHashMap，但在某些情况下，当值或键被强引用时，您必须小心。这可以通过将对象包装在WeakReference中来避免。

import java.lang.ref.WeakReference;
import java.util.HashMap;

public class Test {

    public static void main(String args[]) {
        HashMap<Employee, EmployeeVal> map = 
                      new HashMap<Employee, EmployeeVal>();
        WeakReference<HashMap<Employee, EmployeeVal>> aMap = 
                       new WeakReference<HashMap<Employee, EmployeeVal>>(
                map);

        map = null;

        while (null != aMap.get()) {
            aMap.get().put(new Employee("Vinoth"),
                    new EmployeeVal("Programmer"));
            System.out.println("Size of aMap " + aMap.get().size());
            System.gc();
        }
        System.out.println("Its garbage collected");
    }
}

软引用。

软引用比弱引用强一些。软引用允许垃圾回收，但只有在没有其他选择时才请求垃圾回收器清除它。

The garbage collector does not aggressively collect softly reachable objects the way it does with weakly reachable ones -- instead it only collects softly reachable objects if it really "needs" the memory. Soft references are a way of saying to the garbage collector, "As long as memory isn't too tight, I'd like to keep this object around. But if memory gets really tight, go ahead and collect it and I'll deal with that." The garbage collector is required to clear all soft references before it can throw OutOfMemoryError.

WeakReference:只弱引用的对象将在每个GC周期(minor或full)收集。

SoftReference:当只被软引用的对象被收集时，取决于:

-XX:SoftRefLRUPolicyMSPerMB=N标志(默认值是1000，也就是1秒) 堆中的空闲内存量。 例子: 堆有10MB的空闲空间(在完全GC之后); - xx: SoftRefLRUPolicyMSPerMB = 1000 如果只被SoftReference引用的对象上次被访问的时间大于10秒，则该对象将被收集。