WeakReference与SoftReference
2016-03-21 09:59
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转载自 http://flyneil.iteye.com/blog/1345177
WeakReference与SoftReference都可以用来保存对象的实例引用,这两个类与垃圾回收有关。
WeakReference是弱引用,其中保存的对象实例可以被GC回收掉。这个类通常用于在某处保存对象引用,而又不干扰该对象被GC回收,通常用于Debug、内存监视工具等程序中。因为这类程序一般要求即要观察到对象,又不能影响该对象正常的GC过程。
最近在JDK的Proxy类的实现代码中也发现了Weakrefrence的应用,Proxy会把动态生成的Class实例暂存于一个由Weakrefrence构成的Map中作为Cache。
SoftReference是强引用,它保存的对象实例,除非JVM即将OutOfMemory,否则不会被GC回收。这个特性使得它特别适合设计对象Cache。对于Cache,我们希望被缓存的对象最好始终常驻内存,但是如果JVM内存吃紧,为了不发生OutOfMemoryError导致系统崩溃,必要的时候也允许JVM回收Cache的内存,待后续合适的时机再把数据重新Load到Cache中。这样可以系统设计得更具弹性。
WeakReference的一个测试程序:
程序运行结果:
SoftReference的一个测试程序:
程序运行结果:
另外可参考下面这篇文章
WeakReference,SoftReference 和 PhatomReference 浅析
WeakReference与SoftReference都可以用来保存对象的实例引用,这两个类与垃圾回收有关。
WeakReference是弱引用,其中保存的对象实例可以被GC回收掉。这个类通常用于在某处保存对象引用,而又不干扰该对象被GC回收,通常用于Debug、内存监视工具等程序中。因为这类程序一般要求即要观察到对象,又不能影响该对象正常的GC过程。
最近在JDK的Proxy类的实现代码中也发现了Weakrefrence的应用,Proxy会把动态生成的Class实例暂存于一个由Weakrefrence构成的Map中作为Cache。
SoftReference是强引用,它保存的对象实例,除非JVM即将OutOfMemory,否则不会被GC回收。这个特性使得它特别适合设计对象Cache。对于Cache,我们希望被缓存的对象最好始终常驻内存,但是如果JVM内存吃紧,为了不发生OutOfMemoryError导致系统崩溃,必要的时候也允许JVM回收Cache的内存,待后续合适的时机再把数据重新Load到Cache中。这样可以系统设计得更具弹性。
WeakReference的一个测试程序:
import java.lang.ref.WeakReference; public class WeakReferenceTest { /** * @param args */ public static void main(String[] args) { A a = new A(); a.str = "Hello, reference"; WeakReference<A> weak = new WeakReference<A>(a); a = null; int i = 0; while (weak.get() != null) { System.out.println(String.format("Get str from object of WeakReference: %s, count: %d", weak.get().str, ++i)); if (i % 10 == 0) { System.gc(); System.out.println("System.gc() was invoked!"); } try { Thread.sleep(500); } catch (InterruptedException e) { } } System.out.println("object a was cleared by JVM!"); } }
程序运行结果:
Get str from object of WeakReference: Hello, reference, count: 1 Get str from object of WeakReference: Hello, reference, count: 2 Get str from object of WeakReference: Hello, reference, count: 3 Get str from object of WeakReference: Hello, reference, count: 4 Get str from object of WeakReference: Hello, reference, count: 5 Get str from object of WeakReference: Hello, reference, count: 6 Get str from object of WeakReference: Hello, reference, count: 7 Get str from object of WeakReference: Hello, reference, count: 8 Get str from object of WeakReference: Hello, reference, count: 9 Get str from object of WeakReference: Hello, reference, count: 10 System.gc() was invoked! object a was cleared by JVM!
SoftReference的一个测试程序:
import java.lang.ref.SoftReference; public class SoftReferenceTest { /** * @param args */ public static void main(String[] args) { A a = new A(); a.str = "Hello, reference"; SoftReference<A> sr = new SoftReference<A>(a); a = null; int i = 0; while (sr.get() != null) { System.out.println(String.format("Get str from object of SoftReference: %s, count: %d", sr.get().str, ++i)); if (i % 10 == 0) { System.gc(); System.out.println("System.gc() was invoked!"); } try { Thread.sleep(500); } catch (InterruptedException e) { } } System.out.println("object a was cleared by JVM!"); } }
程序运行结果:
Get str from object of SoftReference: Hello, reference, count: 1 Get str from object of SoftReference: Hello, reference, count: 2 Get str from object of SoftReference: Hello, reference, count: 3 Get str from object of SoftReference: Hello, reference, count: 4 Get str from object of SoftReference: Hello, reference, count: 5 Get str from object of SoftReference: Hello, reference, count: 6 Get str from object of SoftReference: Hello, reference, count: 7 Get str from object of SoftReference: Hello, reference, count: 8 Get str from object of SoftReference: Hello, reference, count: 9 Get str from object of SoftReference: Hello, reference, count: 10 System.gc() was invoked! Get str from object of SoftReference: Hello, reference, count: 11 Get str from object of SoftReference: Hello, reference, count: 12 Get str from object of SoftReference: Hello, reference, count: 13 Get str from object of SoftReference: Hello, reference, count: 14 Get str from object of SoftReference: Hello, reference, count: 15 Get str from object of SoftReference: Hello, reference, count: 16 Get str from object of SoftReference: Hello, reference, count: 17 Get str from object of SoftReference: Hello, reference, count: 18 Get str from object of SoftReference: Hello, reference, count: 19 Get str from object of SoftReference: Hello, reference, count: 20 System.gc() was invoked! Get str from object of SoftReference: Hello, reference, count: 21 Get str from object of SoftReference: Hello, reference, count: 22 Get str from object of SoftReference: Hello, reference, count: 23 Get str from object of SoftReference: Hello, reference, count: 24 Get str from object of SoftReference: Hello, reference, count: 25 Get str from object of SoftReference: Hello, reference, count: 26 Get str from object of SoftReference: Hello, reference, count: 27 Get str from object of SoftReference: Hello, reference, count: 28
另外可参考下面这篇文章
WeakReference,SoftReference 和 PhatomReference 浅析
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