SoftReference(软引用)类是Reference(引用)类的四大子类之一,只被SoftReference(软引用)类对象持有的对象(即被作为所指对象)被称为软可达(softly reachable)对象。在Java中关于软引用的定义是:如果一个对象只存在软引用,则其在空闲内存充裕的情况下不一定会被GC回收,具体情况视回收策略而定。而如果GC回收后空闲内存依旧不足,便会强制回收软可达(softly reachable)对象则会被以避免OOM。虚拟机在抛出OOM错误前会保证清除所有的软可达(softly reachable)对象。
SoftReference(软引用)类适合用于做缓存。软引用适合做缓存是因为其自身可有可无的引用强度决定的,缓存是加快程序的运行效率而非正常运行的必要条件,即使是在没有共存强引用的环境下,只要空闲内存充裕,软引用自身也能够保证热点数据持久存在,这与缓存的运用场景是高度契合的。因此使用SoftReference(软引用)类制作缓存一方面充分利用了空闲内存加快程序的运行效率,另一方面也在空闲内存不足时将仅有的内存资源用于保障程序的正常运行而非浪费在可有可无的地方。
SoftReference(软引用)类继承自Reference(引用)类。当一个SoftReference(软引用)类对象被GC判断该为可回收时,会选择性(会根据回收策略清除其中的一部分)的自动清除清除当前软可达对象及持有其强引用链的软可达对象的所有软引用。
/*** Soft reference objects, which are cleared at the discretion of the garbage* collector in response to memory demand. Soft references are most often used* to implement memory-sensitive caches.* 软引用对象,垃圾收集器根据内存需求自行清除这些对象。软引用最常用于实现对内存敏感的缓存。* * Suppose that the garbage collector determines at a certain point in time* that an object is softly* reachable. At that time it may choose to clear atomically all soft* references to that object and all soft references to any other* softly-reachable objects from which that object is reachable through a chain* of strong references. At the same time or at some later time it will* enqueue those newly-cleared soft references that are registered with* reference queues.* 假设垃圾收集器在某个时间点确定一个对象是软可达的。那时,它会选择性的自动清除对该对象的所有软引用,* 以及对任何其它软可达对象的所有软引用,而该对象可以通过强引用链访问。在同一时间或稍后的某个时间,* 它将对那些注册在引用队列中的新清除的软引用进行入队。*
* All soft references to softly-reachable objects are guaranteed to have* been cleared before the virtual machine throws an* OutOfMemoryError
. Otherwise no constraints are placed upon the* time at which a soft reference will be cleared or the order in which a set* of such references to different objects will be cleared. Virtual machine* implementations are, however, encouraged to bias against clearing* recently-created or recently-used soft references.* 对软可达对象的所有软引用保证在虚拟机抛出OOM之前已被清除。* 虽然对清除软引用的时间或清除对不同对象的这类引用集合的顺序没有任何约束。* 但鼓励虚拟机实现不清除最近创建或最近使用的软引用。*
* Direct instances of this class may be used to implement simple caches;* this class or derived subclasses may also be used in larger data structures* to implement more sophisticated caches. As long as the referent of a soft* reference is strongly reachable, that is, is actually in use, the soft* reference will not be cleared. Thus a sophisticated cache can, for example,* prevent its most recently used entries from being discarded by keeping* strong referents to those entries, leaving the remaining entries to be* discarded at the discretion of the garbage collector.* 该类的直接实例可以用来实现简单的缓存;这个类或派生子类也可以在较大的数据结构中使用,* 以实现更复杂的缓存。只要软引用的所指对象是存在强引用的,也就是说,它实际上正在使用中,软引用就不会被清除。* 因此,一个复杂的缓存可以通过保持对这些条目的强引用来防止最近使用的条目被丢弃,其余的条目则由垃圾收集器自行决定是否丢弃。** @author Mark Reinhold* @since 1.2*/
public class SoftReference extends Reference {...
}
clock(时钟) —— 静态变量,由全局共享。本质是上次GC回收的时间戳,该值由GC负责更新。
/*** Timestamp clock, updated by the garbage collector*/
static private long clock;
timestamp(时间戳) —— 当通过get()方法获取所指对象且所指对象不为null时该值会被更新为clock(时钟)字段值。该值被作为软可达(softly reachable)对象是否被GC回收的判断依据,具体做法是将clock(时钟)字段值减去timestamp(时间戳)字段值的差作为软可达(softly reachable)对象上次使用距离当前时间的近似值,并判断该值是否大于最大间隔,是则回收,否则保留。
/*** Timestamp updated by each invocation of the get method. The VM may use* this field when selecting soft references to be cleared, but it is not* required to do so.*/
private long timestamp;
public SoftReference(T referent) —— 只设置所指对象而不设置引用队列,因此可知SoftReference(软引用)类可不搭配引用队列使用。
/*** Creates a new soft reference that refers to the given object. The new* reference is not registered with any queue.** @param referent object the new soft reference will refer to*/
public SoftReference(T referent) {super(referent);// 时间戳和时钟的初始值都是0。this.timestamp = clock;
}
public SoftReference(T referent, ReferenceQueue super T> q) —— 同时设置所指对象及引用队列。
/*** Creates a new soft reference that refers to the given object and is* registered with the given queue.** @param referent object the new soft reference will refer to* @param q the queue with which the reference is to be registered,* or null if registration is not required*/
public SoftReference(T referent, ReferenceQueue super T> q) {super(referent, q);this.timestamp = clock;
}
public T get() —— 获取所指对象 —— 获取SoftReference(软引用)类对象的所指对象。当所指对象不为null时更新timestamp(时间戳)字段值为clock(时钟)字段值,作为GC判断软可达(softly reachable)对象是否回收的依据。
/*** Returns this reference object's referent. If this reference object has* been cleared, either by the program or by the garbage collector, then* this method returns null
.** @return The object to which this reference refers, or* null
if this reference object has been cleared* @Description: 获取所指对象*/
@Override
public T get() {T o = super.get();// 当所指对象不为空且时间戳不等于时钟时,更新时间戳的值。if (o != null && this.timestamp != clock)this.timestamp = clock;return o;
}
回收策略是在空闲内存充裕的情况下用于回收软可达(softly-reachable)对象的规则。最近最少使用当前堆策略(LRUCurrentHeapPolicy)的全称为Least Recently Used Current Heap Policy,是最常用的回收策略。其思想是只保留使用最为频繁的部分软可达(softly reachable)对象,而将使用不频繁甚至不使用的软可达(softly reachable)对象回收。具体做法是计算软可达(softly reachable)对象的上次获取时间距离当前时间的间隔,并与最大间隔作比较,判断间隔是否小于等于最大间隔,是则保留,否则回收。其中最大间隔 = 空闲堆内存(MB) * JVM参数-XX:SoftRefLRUPolicyMSPerMB(默认值1000ms),故如果假设空闲堆内存为400MB,则可得最大间隔的值为400秒,当软可达(softly reachable)对象的间隔大于400秒时便会被GC回收。具体源码如下:
bool LRUCurrentHeapPolicy::should_clear_reference(oop p, jlong timestamp_clock) {// 使用时钟 - 时间戳得到差值,这个差值可以近似代表软可达(softly reachable)对象上次被获取时间至当前时间的间隔。jlong interval = timestamp_clock - java_lang_ref_SoftReference::timestamp(p);assert(interval >= 0, "Sanity check");// 如果间隔 <= 最大间隔,则不进行回收。if(interval <= _max_interval) {return false;}// 如果间隔 > 最大间隔,则进行回收。return true;
}
void LRUCurrentHeapPolicy::setup() {// 最大间隔 = 空闲堆内存(MB) * 每MB空闲内存保留时间。_max_interval = (Universe::get_heap_free_at_last_gc() / M) * SoftRefLRUPolicyMSPerMB;assert(_max_interval >= 0,"Sanity check");
}
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