// java.lang.Object shell size in bytes: public static final int OBJECT_SHELL_SIZE = 8;
public static final int OBJREF_SIZE = 4; public static final int LONG_FIELD_SIZE = 8; public static final int INT_FIELD_SIZE = 4; public static final int SHORT_FIELD_SIZE = 2; public static final int CHAR_FIELD_SIZE = 2; public static final int BYTE_FIELD_SIZE = 1; public static final int BOOLEAN_FIELD_SIZE = 1; public static final int DOUBLE_FIELD_SIZE = 8; public static final int FLOAT_FIELD_SIZE = 4;
(这些常量不是永远硬编码的,并且对于一个给定的JVM,它们必须独立测量,认识到这一点很重要)当然,幼稚的计算对象域尺寸总和往往忽略了JVM中的存储队列问题。存储队列真的很有关系(例如,Java Tip 130中的初级排列类型),但是我认为在这种低级别的细节上做文章是没有用的。这种细节不但由JVM开发商决定,它们也处在程序员的控制之下。我们的目标是获取对象尺寸的最好估测,并且希望在类域多余、域应该简单组装、或者有必要更紧凑的嵌入数据库等这些时候可以获得提示。为了绝对的物理精度,你可以总是回到Java Tip 130中的Sizeof 类。 为了帮助组成对象实例的配置文件,我们的工具不仅仅计算尺寸,还建立一个附带的有用的数据结构:由IObjectProfileNode组成的图形:
interface IObjectProfileNode { Object object (); String name ();
int size (); int refcount ();
IObjectProfileNode parent (); IObjectProfileNode [] children (); IObjectProfileNode shell ();
IObjectProfileNode [] path (); IObjectProfileNode root (); int pathlength ();
在做了所有这些准备之后,下面就是这种图形遍历的标准实现: public static IObjectProfileNode profile (Object obj) { final IdentityHashMap visited = new IdentityHashMap ();
final ObjectProfileNode root = createProfileTree (obj, visited, CLASS_METADATA_CACHE); finishProfileTree (root);
return root; }
private static ObjectProfileNode createProfileTree (Object obj, IdentityHashMap visited, Map metadataMap) { final ObjectProfileNode root = new ObjectProfileNode (null, obj, null);
final LinkedList queue = new LinkedList ();
queue.addFirst (root); visited.put (obj, root);
final ClassAccessPrivilegedAction caAction = new ClassAccessPrivilegedAction (); final FieldAccessPrivilegedAction faAction = new FieldAccessPrivilegedAction ();
while (! queue.isEmpty ()) { final ObjectProfileNode node = (ObjectProfileNode) queue.removeFirst ();
obj = node.m_obj; final Class objClass = obj.getClass ();
if (objClass.isArray ()) { final int arrayLength = Array.getLength (obj); final Class componentType = objClass.getComponentType ();
// Add shell pseudo-node: final AbstractShellProfileNode shell = new ArrayShellProfileNode (node, objClass, arrayLength); shell.m_size = sizeofArrayShell (arrayLength, componentType);
node.m_shell = shell; node.addFieldRef (shell);
if (! componentType.isPrimitive ()) { // Traverse each array slot: for (int i = 0; i < arrayLength; ++ i) { final Object ref = Array.get (obj, i);
if (ref != null) { ObjectProfileNode child = (ObjectProfileNode) visited.get (ref); if (child != null) ++ child.m_refcount; else { child = new ObjectProfileNode (node, ref, new ArrayIndexLink (node.m_link, i)); node.addFieldRef (child);
queue.addLast (child); visited.put (ref, child); } } } } } else // the object is of a non-array type { final ClassMetadata metadata = getClassMetadata (objClass, metadataMap, caAction, faAction); final Field [] fields = metadata.m_refFields;
// Add shell pseudo-node: final AbstractShellProfileNode shell = new ObjectShellProfileNode (node, metadata.m_primitiveFieldCount, metadata.m_refFields.length); shell.m_size = metadata.m_shellSize;
node.m_shell = shell; node.addFieldRef (shell);
// Traverse all non-null ref fields: for (int f = 0, fLimit = fields.length; f < fLimit; ++ f) { final Field field = fields [f];
final Object ref; try // to get the field value: { ref = field.get (obj); } catch (Exception e) { throw new RuntimeException ("cannot get field [" + field.getName () + "] of class [" + field.getDeclaringClass ().getName () + "]: " + e.toString ()); }
if (ref != null) { ObjectProfileNode child = (ObjectProfileNode) visited.get (ref); if (child != null) ++ child.m_refcount; else { child = new ObjectProfileNode (node, ref, new ClassFieldLink (field)); node.addFieldRef (child);
该代码是上一篇Java Q&A, "Attack of the Clones."使用的"通过反射克隆"实现的远亲。如前所述,它缓存了反射元数据来提高性能,并且使用了一个标识散列映射来标记访问过的对象。profile()方法从宽度优先遍历中的具有IObjectProfileNode的生成树的原始对象图形开始,以合计和分配所有节点尺寸的快速后序遍历结束。profile()返回一个 IObjectProfileNode,即产生的生成树的根,它的尺寸就是整个图形的尺寸。 当然, profile()的输出只有当我有一个很好的方法扩展它时才有用。为了这个目的,每个IObjectProfileNode 必须支持由节点访问者和节点过滤器一起进行的测试: