java垃圾收集器 - “获取”已删除的对象

Question

is there a possibility to see which objects will be removed with the garbage collector? I do not need the content of the object, but the class of the object is necessary.

I try to write a real time application, which creates and deletes a lot of objects and after a while the application slow down. At the moment I'm not sure whether it's a problem of my code or from the external libraries. So perfect would be an output, which identifies all classes, which has been removed, together with their "count" (how many of this objects has been removed).

I hope somebody can help me.

Best, Michael

Answer 1

You could try monitoring your application with VisualVM. There is a plugin that gives info on the garbage collector activity.

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

Answer 2

How about overriding the finalize method for your objects and logging the name of the class there? Beware though, this might prevent the object from being garbage collected.

Your class might look something like this:

public class MyObject {

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

Here is the Object.finalize() method signature and documentation.

/**
 * 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

Wouldn't the objects that are not garbage collected be even more interesting?

Anway, a memory profiler like VisualVM is what you really need. It can show you exactly how many objects of each class exist in your application, and it can also identify garbage collected classes and object county by comparing heap dumps before and after GC.

Answer 4

我建议你使用任何java profilers，如JProfiler，Yourkit等。很容易看到垃圾收集的对象数量以及对象的类型。

Answer 5

The thread for garbage collection runs on low priority. So this thread is not getting its turn to do the cleanup task.

Also there is not guarantee that the garbage collection thread will run always. Here in your application the low priority thread of garbage collection is not getting chance to execute over the application threads. So the heap is not getting cleared from unused objects and hence application slows down due to limited heap size.

You can count the number of objects garbage collected by overriding public void finalize() method. Check javadoc for more details

I would recommend you to use some java profilers to diagnose memory issue.