Sizeof


1   // From http://www.javaworld.com/javatips/jw-javatip130_p.html
2   
3   /**
4    * A simple class to experiment with your JVM's garbage collector
5    * and memory sizes for various data types.
6    *
7    * @author <a HREF="mailto:vlad@trilogy.com">Vladimir Roubtsov</a>
8    */
9   public class Sizeof {
10    public static void main(String   [] args) throws Exception   {
11      // "warm up" all classes/methods that we are going to use:
12      runGC();
13      usedMemory();
14          
15      // array to keep strong references to allocated objects:
16      final int count = 10000; // 10000 or so is enough for small ojects
17      Object   [] objects = new Object   [count];
18          
19      long heap1 = 0;
20  
21      // allocate count+1 objects, discard the first one:
22      for (int i = -1; i < count; ++ i) {
23        Object   object;
24              
25        // INSTANTIATE YOUR DATA HERE AND ASSIGN IT TO 'object':
26              
27        object = new Object  (); // 8 bytes
28        //object = new Integer(i); // 16 bytes
29        //object = new Long(i); // same size as Integer?
30        //object = createString(10); // 56 bytes? fine...
31        //object = createString(9)+' '; // 72 bytes? the article explains why
32        //object = new char [10]; // 32 bytes
33        //object = new byte [32][1]; // 656 bytes?!
34            
35        if (i >= 0)
36          objects [i] = object;
37        else {
38          object = null; // discard the "warmup" object
39          runGC();
40          heap1 = usedMemory(); // take a "before" heap snapshot
41        }
42      }
43  
44      runGC();
45      long heap2 = usedMemory(); // take an "after" heap snapshot:
46          
47      final int size = Math.round(((float)(heap2 - heap1))/count);
48      System.out.println("'before' heap: " + heap1 +
49                          ", 'after' heap: " + heap2);
50      System.out.println("heap delta: " + (heap2 - heap1) +
51          ", {" + objects [0].getClass() + "} size = " + size + " bytes");
52    }
53      
54    // a helper method for creating Strings of desired length
55    // and avoiding getting tricked by String interning:
56    public static String   createString(final int length) {
57      final char [] result = new char [length];
58      for (int i = 0; i < length; ++ i)
59        result [i] = (char) i;
60          
61      return new String  (result);
62    }
63  
64    // this is our way of requesting garbage collection to be run:
65    // [how aggressive it is depends on the JVM to a large degree, but
66    // it is almost always better than a single Runtime.gc() call]
67    private static void runGC() throws Exception   {
68      // for whatever reason it helps to call Runtime.gc()
69      // using several method calls:
70      for (int r = 0; r < 4; ++ r)
71        _runGC();
72    }
73  
74    private static void _runGC() throws Exception   {
75      long usedMem1 = usedMemory(), usedMem2 = Long.MAX_VALUE;
76  
77      for (int i = 0; (usedMem1 < usedMem2) && (i < 1000); ++ i) {
78        s_runtime.runFinalization();
79        s_runtime.gc();
80        Thread.currentThread().yield();
81              
82        usedMem2 = usedMem1;
83        usedMem1 = usedMemory();
84      }
85    }
86  
87    private static long usedMemory() {
88      return s_runtime.totalMemory() - s_runtime.freeMemory();
89    }
90      
91    private static final Runtime   s_runtime = Runtime.getRuntime();
92  }
93
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