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UID ibm Contact and feedback Need support? Let's first define what it means for an object to be alive a. To define these terms, there's no way around defining references and reachability, so we quickly get into some subtle and slightly confusing concepts; alas, let's dive in. Starting with the basics, a Java heap is a directed graph of objects and their references:. The blue boxes represent live garbage collection roots a. GC roots or heap roots.
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GC roots may only be specific types of objects such as system classloaders and classes, threads, thread stack locals, JNI references, etc. At the simplest level, a directed arrow in the picture above represents a reference from one object to another object the non-directed lines between the GC roots simply represent that the GC roots are part of the same graph.
For example, imagine that the class of Object1 is the following:. In the above picture of Object1, the firstName and lastName fields have references to instances of java. String Object1a and Object1b are those Strings, respectively :.
At the simplest level, garbage collection starts at the live GC roots and walks all reachable paths to referenced objects and this is called the mark phase. Any objects reachable from live GC roots are marked as well as the live GC roots themselves.
Any unmarked objects that remain are garbage. The sweep phase of garbage collection may sweep away such garbage objects from the heap not every garbage collection necessarily sweeps away all garbage.
In the first picture at the top of this article, GarbageObject1 is not reachable from any of the GC roots and thus it may be swept away by the next garbage collection. Note that some garbage collectors are very sophisticated and mark and sweep subset s of the heap at any one time to reduce worst-case garbage collection pause times.
There are different types of reachability : let's start with the simplest one — strongly reachable — which we have been implicitly discussing above:. An object is strongly reachable if it can be reached by some thread without traversing any reference objects. A newly-created object is strongly reachable by the thread that created it. In other words, if none of the objects on the path between a thread GC root and the target object are an instance of a subclass of java.
Referencethen an object is strongly reachable. Now let's discuss the other types of reachability. Why might java. Reference be useful? The simplest example is a java. SoftReferences are great for transient caches because they may be used to keep an object around "softly" — if the JVM really needs the space and an object is only softly reachable, then the JVM may consider it as garbage.
For example, let's extend our Person class above to have a SoftReference to a byte array that represents the person's profile picture:.
The above design caches the database lookup of a person's profile picture into a SoftReference field when getProfilePicture is called.
If there is a subsequent call to getProfilePicture for this instance of Person, and if the Java heap is not under memory pressure, then the database lookup is avoided. At any point in time, the garbage collector may decide to clear the byte array within the SoftReference and then the code will re-do the database lookup.
Here's how the object graph might look like if the SoftReference is set:. Another type of java. Reference is a java. PhantomReferences are just a more flexible form of Java finalization. Java finalization and PhantomReferences may be used to perform processing of an object after it is essentially garbage but before it has been fully collected by the garbage collector.
In general, finalization and PhantomReferences are discouraged because the processing of such objects is generally non-deterministic which may lead to native memory leaks, native OOMs, and garbage collection pause time variability especially with generational garbage collectors, which most modern garbage collectors are, because such objects may build up in the older generations and garbage collection may not process them for a long time.
The final type of java. WeakReferences are just like SoftReferences, except that WeakReferences are collected eagerly, whereas SoftReferences are not collected eagerly as long as there is plenty of free Java heap.
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Now that we have a clear picture of reachability and GC roots, we can use MAT to analyze a heapdump and find the paths to GC roots that are keeping suspect objects alive.
One of the most common types of GC roots is a thread stack frame local, so although you may find your suspect rooted in such a GC root, a PHD won't tell you which thread that root is on, so it may be difficult to understand what that thread was doing.
Therefore, in general, and particularly for OOMs, system dumps are preferred over PHDs particularly because they have more accurate GC roots note that system dumps, like HPROF dumps, include all Java memory content such as String and primitive values, so they should be treated sensitively.
In general, we are not interested in phantom, weak, or soft reference paths to suspect objects, so this option excludes such paths. Here is an example result:. In this example, we ran this query on the selection of byte arrays. The results of the query will list the GC roots and then you expand the nodes until you reach your selected objects. The way to read this table is the same as in the first picture in this article: The GC root has a reference to the child tree element and so on all the way to the selected objects.
In this example, the shortest path to any GC root for these byte arrays starts at the static LauncherHelper class which is a GC root. This class has a static field called holder which is an ArrayList.