java垃圾收集器 - 「獲取」已刪除的對象

Question

有沒有可能看到哪些對象將被垃圾收集器刪除？我不需要該對象的內容，但該對象的類是必要的。

我嘗試編寫一個實時應用程序，它創建並刪除了很多對象，過了一段時間後應用程序變慢了。目前我不確定這是我的代碼還是來自外部庫的問題。所以完美的是一個輸出，它標識所有已經被刪除的類以及它們的「數量」（有多少個這些對象已被刪除）。

我希望有人能幫助我。

最佳， 邁克爾

Answer 1

您可以嘗試使用VisualVM監視您的應用程序。有一個插件可以提供有關垃圾收集器活動的信息。

http://visualvm.java.net/plugins.html

Answer 2

如何重寫finalize方法爲你的對象和記錄類有名字？但要小心，這可能會阻止垃圾收集對象。

你的類可能是這個樣子：

public class MyObject { 

    @Override 
    public void finalize() throws Throwable { 
     logger.debug("Object of class {} being garbage collected", this.getClass().getName()); 
    } 
} 

這裏是Object.finalize()方法簽名和文檔。

/** 
* Called by the garbage collector on an object when garbage collection 
* determines that there are no more references to the object. 
* A subclass overrides the <code>finalize</code> method to dispose of 
* system resources or to perform other cleanup. 
* <p> 
* The general contract of <tt>finalize</tt> is that it is invoked 
* if and when the Java<font size="-2"><sup>TM</sup></font> virtual 
* machine has determined that there is no longer any 
* means by which this object can be accessed by any thread that has 
* not yet died, except as a result of an action taken by the 
* finalization of some other object or class which is ready to be 
* finalized. The <tt>finalize</tt> method may take any action, including 
* making this object available again to other threads; the usual purpose 
* of <tt>finalize</tt>, however, is to perform cleanup actions before 
* the object is irrevocably discarded. For example, the finalize method 
* for an object that represents an input/output connection might perform 
* explicit I/O transactions to break the connection before the object is 
* permanently discarded. 
* <p> 
* The <tt>finalize</tt> method of class <tt>Object</tt> performs no 
* special action; it simply returns normally. Subclasses of 
* <tt>Object</tt> may override this definition. 
* <p> 
* The Java programming language does not guarantee which thread will 
* invoke the <tt>finalize</tt> method for any given object. It is 
* guaranteed, however, that the thread that invokes finalize will not 
* be holding any user-visible synchronization locks when finalize is 
* invoked. If an uncaught exception is thrown by the finalize method, 
* the exception is ignored and finalization of that object terminates. 
* <p> 
* After the <tt>finalize</tt> method has been invoked for an object, no 
* further action is taken until the Java virtual machine has again 
* determined that there is no longer any means by which this object can 
* be accessed by any thread that has not yet died, including possible 
* actions by other objects or classes which are ready to be finalized, 
* at which point the object may be discarded. 
* <p> 
* The <tt>finalize</tt> method is never invoked more than once by a Java 
* virtual machine for any given object. 
* <p> 
* Any exception thrown by the <code>finalize</code> method causes 
* the finalization of this object to be halted, but is otherwise 
* ignored. 
* 
* @throws Throwable the <code>Exception</code> raised by this method 
*/ 
protected void finalize() throws Throwable { } 

Answer 3

豈不是不垃圾回收的對象更加有趣？

Anway，像VisualVM這樣的內存分析器就是你真正需要的。它可以準確顯示您的應用程序中存在每個類的對象數量，還可以通過比較GC之前和之後的堆轉儲來識別垃圾收集的類和對象縣。

Answer 4

我建議你使用任何java profileor，比如JProfiler，Yourkit等。 很容易看到垃圾收集對象的數量以及對象的類型。

Answer 5

垃圾收集線程運行在低優先級。所以這個線程沒有輪到做清理任務。

此外，不保證垃圾收集線程將始終運行。在你的應用程序中，垃圾收集的低優先級線程沒有機會在應用程序線程上執行。所以堆不會從未使用的對象中清除，因此應用程序由於堆大小有限而變慢。

您可以通過重寫public void finalize()方法來計算垃圾收集對象的數量。檢查javadoc瞭解更多詳情

我建議你使用一些java分析器來診斷內存問題。