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Java Objects size estimation.
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package com.carrotsearch.sizeof;

import java.io.PrintWriter;
import java.io.StringWriter;
import java.lang.reflect.*;
import java.util.*;

import com.carrotsearch.sizeof.RamUsageEstimator;

/**
 * Dumps retained and shallow memory taken by an object and its reference
 * hierarchy. This could be improved in a number of ways but works for debugging
 * and "live" object monitoring. For static heap analyses a real performance
 * monitor like YourKit is much better (and accurate?).
 * 
 * @see #dump(PrintWriter, Object)
 * @see #dump(Object)
 */
public class ObjectTree {
  /**
   * Dump the object tree to a {@link PrintWriter}.
   */
  public static void dump(PrintWriter pw, Object root) {
    Node nodeTree = Node.create(root);
    printTree(new StringBuilder(), new StringBuilder(), pw, nodeTree);
  }

  /**
   * Dump the object tree to a {@link String}.
   */
  public static String dump(Object root) {
    StringWriter sw = new StringWriter();
    PrintWriter pw = new PrintWriter(sw);
    dump(pw, root);
    pw.flush();
    return sw.toString();
  }
  
  private static class Node {
    private String name;
    private List children;
    
    private long shallowSize;
    private long deepSize;
    
    public Node(String name, Object delegate) {
      this.name = name;
      
      if (delegate != null) {
        shallowSize = RamUsageEstimator.shallowSizeOf(delegate);
        deepSize = shallowSize;
      }
    }
    
    private void addChild(Node node) {
      if (children == null) {
        children = new ArrayList();
      }
      children.add(node);
      deepSize += node.deepSize;
    }
    
    /** Factory create method. */
    public static Node create(Object delegate) {
      return create("root", delegate, new IdentityHashMap());
    }
    
    /**
     * Factory create method.
     */
    public static Node create(String prefix, Object delegate,
        IdentityHashMap seen) {
      if (delegate == null) throw new IllegalArgumentException();
      
      if (seen.containsKey(delegate)) {
        return new Node("[seen " + uniqueName(delegate, seen) + "]", null);
      }
      seen.put(delegate, seen.size());
      
      Class clazz = delegate.getClass();
      if (clazz.isArray()) {
        Node parent = new Node(prefix + " => " + clazz.getSimpleName(),
            delegate);
        if (clazz.getComponentType().isPrimitive()) {
          return parent;
        } else {
          final int length = Array.getLength(delegate);
          for (int i = 0; i < length; i++) {
            Object value = Array.get(delegate, i);
            if (value != null) {
              parent.addChild(create("[" + i + "]", value, seen));
            }
          }
          return parent;
        }
      } else {
        List declaredFields = new ArrayList();
        for (Class c = clazz; c != null; c = c.getSuperclass()) {
          Field[] fields = c.getDeclaredFields();
          AccessibleObject.setAccessible(fields, true);
          declaredFields.addAll(Arrays.asList(fields));
        }
        Collections.sort(declaredFields, new Comparator() {
          @Override
          public int compare(Field o1, Field o2) {
            return o1.getName().compareTo(o2.getName());
          }
        });
        
        Node parent = new Node(prefix + " => " + uniqueName(delegate, seen),
            delegate);
        for (Field f : declaredFields) {
          try {
            if (!Modifier.isStatic(f.getModifiers())
                && !f.getType().isPrimitive()) {
              Object fValue = f.get(delegate);
              if (fValue != null) {
                parent.addChild(create(
                    f.getType().getSimpleName() + " " + f.getName(), fValue,
                    seen));
              } else {
                parent.addChild(new Node(f.getType().getSimpleName() + " "
                    + f.getName() + " => null", null));
              }
            }
          } catch (Exception e) {
            throw new RuntimeException(e);
          }
        }
        return parent;
      }
    }
    
    private static String uniqueName(Object t,
        IdentityHashMap seen) {
      return "<" + t.getClass().getSimpleName() + "#" + seen.get(t) + ">";
    }
    
    public String getName() {
      return name;
    }
    
    public boolean hasChildren() {
      return children != null && !children.isEmpty();
    }
    
    public List getChildren() {
      return children;
    }
  }
  
  private static void printTree(StringBuilder prefix, StringBuilder line,
      PrintWriter pw, Node node) {
    line.append(node.getName());
    pw.println(String.format(Locale.ENGLISH, "%,8d %,8d  %s", node.deepSize,
        node.shallowSize, line.toString()));
    line.setLength(0);
    
    if (node.hasChildren()) {
      int pLen = prefix.length();
      for (Iterator i = node.getChildren().iterator(); i.hasNext();) {
        Node next = i.next();
        line.append(prefix.toString());
        line.append("+- ");
        prefix.append(i.hasNext() ? "|  " : "   ");
        printTree(prefix, line, pw, next);
        prefix.setLength(pLen);
      }
    }
  }
}