This section provides a tutorial example on how to search for an object instance and browse its values and references in a Java heap dump file through the 'jhat' Web server.
After looking object instance counts, I want to find some object instances
created by my program and browse their values.
My program, GarbageCollection.java, used the following code
to create many java.lang.Object[] instances:
private static Object getOneMega() {
Object[] lst = new Object[10];
lst[0] = new long[256*128];
lst[1] = new int[256*256];
lst[2] = new double[256*128];
lst[3] = new float[64*256];
lst[4] = new byte[64*1024];
String[] l = new String[64*64];
for (int i=0; i<64*64; i++)
l[i] = new String("12345678"); // 16B
lst[5] = l;
lst[6] = new char[64*512];
return lst;
}
I want to find one of these Object[] instances and review its elements.
1. Run a Web browser with http://localhost:7000. The heap dump first page shows up.
2. Click the link of "Show instance counts for all classes (including platform)".
The instance count page shows up.
3. Click the link of "instances" in the line of "330 instances of class [Ljava.lang.Object;".
Remember that "[Ljava.lang.Object;" is the class name for Object[] arrays.
A list of all 330 instances is displayed with their addresses.
3. Click on the first instance of "[Ljava.lang.Object;@0x25a9e6d0 (48 bytes)".
The contents of this instance show up:
Object at 0x25a9e6d0
Array of 10 objects
Class:
class [Ljava.lang.Object;
Values
0 : [J@0x25aaea00 (262152 bytes)
1 : [I@0x25aeea10 (262152 bytes)
2 : [D@0x25b2ea20 (262152 bytes)
3 : [F@0x25b6ea30 (65544 bytes)
4 : [B@0x25b7ea40 (65544 bytes)
5 : [Ljava.lang.String;@0x25b8ea50 (16392 bytes)
6 : [C@0x25baa7b0 (65544 bytes)
7 : null
8 : null
9 : null
References to this object:
[Ljava.lang.Object;@0x2536e920 (72 bytes)
: Element 8 of [Ljava.lang.Object;@0x2536e920
Excellent! I am lucky that the first instance of Object[] is created by my code.
"jhat" does a good job showing me everything about this instance.
I can continue clicking its values or references to get information.
Conclusion: "jhat" is much easier to use than many Java debugger for browser heap objects.
