tools/ahat/src/main/com/android/ahat/heapdump/AhatClassInstance.java - platform/art - Git at Google

 /*
  * Copyright (C) 2016 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 package com.android.ahat.heapdump;

 import java.awt.image.BufferedImage;
 import java.util.Iterator;
 import java.util.NoSuchElementException;

 /**
  * A typical Java object from a parsed heap dump.
  * Note that this is used for Java objects that are instances of classes (as
  * opposed to arrays), not for class objects themselves.
  * See {@link AhatClassObj } for the representation of class objects.
  * <p>
  * This class provides a method for iterating over the instance fields of the
  * object in addition to those methods inherited from {@link AhatInstance}.
  */
 public class AhatClassInstance extends AhatInstance {
   // Instance fields of the object. These are stored in order of the instance
   // field descriptors from the class object, starting with this class first,
   // followed by the super class, and so on. We store the values separate from
   // the field types and names to save memory.
   private Value[] mFields;

   AhatClassInstance(long id) {
     super(id);
   }

   void initialize(Value[] fields) {
     mFields = fields;
   }

   @Override
   long getExtraJavaSize() {
     return 0;
   }

   @Override public Value getField(String fieldName) {
     for (FieldValue field : getInstanceFields()) {
       if (fieldName.equals(field.name)) {
         return field.value;
       }
     }
     return null;
   }

   @Override public AhatInstance getRefField(String fieldName) {
     Value value = getField(fieldName);
     return value == null ? null : value.asAhatInstance();
   }

   /**
    * Read an int field of an instance.
    * The field is assumed to be an int type.
    * Returns <code>def</code> if the field value is not an int or could not be
    * read.
    */
   private Integer getIntField(String fieldName, Integer def) {
     Value value = getField(fieldName);
     if (value == null || !value.isInteger()) {
       return def;
     }
     return value.asInteger();
   }

   /**
    * Read a long field of this instance.
    * The field is assumed to be a long type.
    * Returns <code>def</code> if the field value is not an long or could not
    * be read.
    */
   private Long getLongField(String fieldName, Long def) {
     Value value = getField(fieldName);
     if (value == null || !value.isLong()) {
       return def;
     }
     return value.asLong();
   }

   /**
    * Returns the list of class instance fields for this instance.
    * Includes values of field inherited from the superclass of this instance.
    * The fields are returned in no particular order.
    *
    * @return Iterable over the instance field values.
    */
   public Iterable<FieldValue> getInstanceFields() {
     return new InstanceFieldIterator(mFields, getClassObj());
   }

   @Override
   Iterable<Reference> getReferences() {
     return new ReferenceIterator();
   }

   @Override public String asString(int maxChars) {
     if (!isInstanceOfClass("java.lang.String")) {
       return null;
     }

     Value value = getField("value");
     if (value == null || !value.isAhatInstance()) {
       return null;
     }

     AhatInstance inst = value.asAhatInstance();
     if (inst.isArrayInstance()) {
       AhatArrayInstance chars = inst.asArrayInstance();
       int numChars = chars.getLength();
       int count = getIntField("count", numChars);
       int offset = getIntField("offset", 0);
       return chars.asMaybeCompressedString(offset, count, maxChars);
     }
     return null;
   }

   @Override public AhatInstance getReferent() {
     if (isInstanceOfClass("java.lang.ref.Reference")) {
       return getRefField("referent");
     }
     return null;
   }

   @Override public String getDexCacheLocation(int maxChars) {
     if (isInstanceOfClass("java.lang.DexCache")) {
       AhatInstance location = getRefField("location");
       if (location != null) {
         return location.asString(maxChars);
       }
     }
     return null;
   }

   @Override public String getBinderProxyInterfaceName() {
     if (isInstanceOfClass("android.os.BinderProxy")) {
       for (AhatInstance inst : getReverseReferences()) {
         String className = inst.getClassName();
         if (className.endsWith("$Stub$Proxy")) {
           Value value = inst.getField("mRemote");
           if (value != null && value.asAhatInstance() == this) {
             return className.substring(0, className.lastIndexOf("$Stub$Proxy"));
           }
         }
       }
     }
     return null;
   }

   @Override public String getBinderTokenDescriptor() {
     String descriptor = getBinderDescriptor();
     if (descriptor == null) {
       return null;
     }

     if (isInstanceOfClass(descriptor + "$Stub")) {
       // This is an instance of an auto-generated interface class, and
       // therefore not a binder token.
       return null;
     }

     return descriptor;
   }

   @Override public String getBinderStubInterfaceName() {
     String descriptor = getBinderDescriptor();
     if (descriptor == null || descriptor.isEmpty()) {
       // Binder interface stubs always have a non-empty descriptor
       return null;
     }

     // We only consider something a binder service if it's an instance of the
     // auto-generated descriptor$Stub class.
     if (isInstanceOfClass(descriptor + "$Stub")) {
       return descriptor;
     }

     return null;
   }

   @Override public AhatInstance getAssociatedBitmapInstance() {
     return getBitmapInfo() == null ? null : this;
   }

   @Override public boolean isClassInstance() {
     return true;
   }

   @Override public AhatClassInstance asClassInstance() {
     return this;
   }

   @Override public String toString() {
     return String.format("%s@%08x", getClassName(), getId());
   }

   /**
    * Returns the descriptor of an android.os.Binder object.
    * If no descriptor is set, returns an empty string.
    * If the object is not an android.os.Binder object, returns null.
    */
   private String getBinderDescriptor() {
     if (isInstanceOfClass("android.os.Binder")) {
       Value value = getField("mDescriptor");;

       if (value == null) {
         return "";
       } else {
         return value.asAhatInstance().asString();
       }
     } else {
       return null;
     }
   }

   /**
    * Read the given field from the given instance.
    * The field is assumed to be a byte[] field.
    * Returns null if the field value is null, not a byte[] or could not be read.
    */
   private byte[] getByteArrayField(String fieldName) {
     AhatInstance field = getRefField(fieldName);
     return field == null ? null : field.asByteArray();
   }

   private static class BitmapInfo {
     public final int width;
     public final int height;
     public final byte[] buffer;

     public BitmapInfo(int width, int height, byte[] buffer) {
       this.width = width;
       this.height = height;
       this.buffer = buffer;
     }
   }

   /**
    * Return bitmap info for this object, or null if no appropriate bitmap
    * info is available.
    */
   private BitmapInfo getBitmapInfo() {
     if (!isInstanceOfClass("android.graphics.Bitmap")) {
       return null;
     }

     Integer width = getIntField("mWidth", null);
     if (width == null) {
       return null;
     }

     Integer height = getIntField("mHeight", null);
     if (height == null) {
       return null;
     }

     byte[] buffer = getByteArrayField("mBuffer");
     if (buffer == null) {
       return null;
     }

     if (buffer.length < 4 * height * width) {
       return null;
     }

     return new BitmapInfo(width, height, buffer);

   }

   @Override public BufferedImage asBitmap() {
     BitmapInfo info = getBitmapInfo();
     if (info == null) {
       return null;
     }

     // Convert the raw data to an image
     // Convert BGRA to ABGR
     int[] abgr = new int[info.height * info.width];
     for (int i = 0; i < abgr.length; i++) {
       abgr[i] = (
           (((int) info.buffer[i * 4 + 3] & 0xFF) << 24)
           + (((int) info.buffer[i * 4 + 0] & 0xFF) << 16)
           + (((int) info.buffer[i * 4 + 1] & 0xFF) << 8)
           + ((int) info.buffer[i * 4 + 2] & 0xFF));
     }

     BufferedImage bitmap = new BufferedImage(
         info.width, info.height, BufferedImage.TYPE_4BYTE_ABGR);
     bitmap.setRGB(0, 0, info.width, info.height, abgr, 0, info.width);
     return bitmap;
   }

   @Override
   RegisteredNativeAllocation asRegisteredNativeAllocation() {
     if (!isInstanceOfClass("sun.misc.Cleaner")) {
       return null;
     }

     Value vthunk = getField("thunk");
     if (vthunk == null || !vthunk.isAhatInstance()) {
       return null;
     }

     AhatClassInstance thunk = vthunk.asAhatInstance().asClassInstance();
     if (thunk == null
         || !thunk.isInstanceOfClass("libcore.util.NativeAllocationRegistry$CleanerThunk")) {
       return null;
     }

     Value vregistry = thunk.getField("this$0");
     if (vregistry == null || !vregistry.isAhatInstance()) {
       return null;
     }

     AhatClassInstance registry = vregistry.asAhatInstance().asClassInstance();
     if (registry == null || !registry.isInstanceOfClass("libcore.util.NativeAllocationRegistry")) {
       return null;
     }

     Value size = registry.getField("size");
     if (!size.isLong()) {
       return null;
     }

     Value referent = getField("referent");
     if (referent == null || !referent.isAhatInstance()) {
       return null;
     }

     RegisteredNativeAllocation rna = new RegisteredNativeAllocation();
     rna.referent = referent.asAhatInstance();
     rna.size = size.asLong();
     return rna;
   }

   private static class InstanceFieldIterator implements Iterable<FieldValue>,
                                                         Iterator<FieldValue> {
     // The complete list of instance field values to iterate over, including
     // superclass field values.
     private Value[] mValues;
     private int mValueIndex;

     // The list of field descriptors specific to the current class in the
     // class hierarchy, not including superclass field descriptors.
     // mFields and mFieldIndex are reset each time we walk up to the next
     // superclass in the call hierarchy.
     private Field[] mFields;
     private int mFieldIndex;
     private AhatClassObj mNextClassObj;

     public InstanceFieldIterator(Value[] values, AhatClassObj classObj) {
       mValues = values;
       mFields = classObj.getInstanceFields();
       mValueIndex = 0;
       mFieldIndex = 0;
       mNextClassObj = classObj.getSuperClassObj();
     }

     @Override
     public boolean hasNext() {
       // If we have reached the end of the fields in the current class,
       // continue walking up the class hierarchy to get superclass fields as
       // well.
       while (mFieldIndex == mFields.length && mNextClassObj != null) {
         mFields = mNextClassObj.getInstanceFields();
         mFieldIndex = 0;
         mNextClassObj = mNextClassObj.getSuperClassObj();
       }
       return mFieldIndex < mFields.length;
     }

     @Override
     public FieldValue next() {
       if (!hasNext()) {
         throw new NoSuchElementException();
       }
       Field field = mFields[mFieldIndex++];
       Value value = mValues[mValueIndex++];
       return new FieldValue(field.name, field.type, value);
     }

     @Override
     public Iterator<FieldValue> iterator() {
       return this;
     }
   }

   /**
    * Returns the reachability type associated with this instance.
    * For example, returns Reachability.WEAK for an instance of
    * java.lang.ref.WeakReference.
    */
   private Reachability getJavaLangRefType() {
     AhatClassObj cls = getClassObj();
     while (cls != null) {
       switch (cls.getName()) {
         case "java.lang.ref.PhantomReference": return Reachability.PHANTOM;
         case "java.lang.ref.WeakReference": return Reachability.WEAK;
         case "java.lang.ref.FinalizerReference": return Reachability.FINALIZER;
         case "java.lang.ref.Finalizer": return Reachability.FINALIZER;
         case "java.lang.ref.SoftReference": return Reachability.SOFT;
       }
       cls = cls.getSuperClassObj();
     }
     return Reachability.STRONG;
   }

   /**
    * A Reference iterator that iterates over the fields of this instance.
    */
   private class ReferenceIterator implements Iterable<Reference>,
                                              Iterator<Reference> {
     private final Iterator<FieldValue> mIter = getInstanceFields().iterator();
     private Reference mNext = null;

     // If we are iterating over a subclass of java.lang.ref.Reference, the
     // 'referent' field doesn't have strong reachability. mJavaLangRefType
     // describes what type of java.lang.ref.Reference subinstance this is.
     private final Reachability mJavaLangRefType = getJavaLangRefType();

     @Override
     public boolean hasNext() {
       while (mNext == null && mIter.hasNext()) {
         FieldValue field = mIter.next();
         if (field.value != null && field.value.isAhatInstance()) {
           Reachability reachability = Reachability.STRONG;
           if (mJavaLangRefType != Reachability.STRONG && "referent".equals(field.name)) {
             reachability = mJavaLangRefType;
           }
           AhatInstance ref = field.value.asAhatInstance();
           mNext = new Reference(AhatClassInstance.this, "." + field.name, ref, reachability);
         }
       }
       return mNext != null;
     }

     @Override
     public Reference next() {
       if (!hasNext()) {
         throw new NoSuchElementException();
       }
       Reference next = mNext;
       mNext = null;
       return next;
     }

     @Override
     public Iterator<Reference> iterator() {
       return this;
     }
   }
 }
	/*
	* Copyright (C) 2016 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	package com.android.ahat.heapdump;

	import java.awt.image.BufferedImage;
	import java.util.Iterator;
	import java.util.NoSuchElementException;

	/**
	* A typical Java object from a parsed heap dump.
	* Note that this is used for Java objects that are instances of classes (as
	* opposed to arrays), not for class objects themselves.
	* See {@link AhatClassObj } for the representation of class objects.
	* <p>
	* This class provides a method for iterating over the instance fields of the
	* object in addition to those methods inherited from {@link AhatInstance}.
	*/
	public class AhatClassInstance extends AhatInstance {
	// Instance fields of the object. These are stored in order of the instance
	// field descriptors from the class object, starting with this class first,
	// followed by the super class, and so on. We store the values separate from
	// the field types and names to save memory.
	private Value[] mFields;

	AhatClassInstance(long id) {
	super(id);
	}

	void initialize(Value[] fields) {
	mFields = fields;
	}

	@Override
	long getExtraJavaSize() {
	return 0;
	}

	@Override public Value getField(String fieldName) {
	for (FieldValue field : getInstanceFields()) {
	if (fieldName.equals(field.name)) {
	return field.value;
	}
	}
	return null;
	}

	@Override public AhatInstance getRefField(String fieldName) {
	Value value = getField(fieldName);
	return value == null ? null : value.asAhatInstance();
	}

	/**
	* Read an int field of an instance.
	* The field is assumed to be an int type.
	* Returns <code>def</code> if the field value is not an int or could not be
	* read.
	*/
	private Integer getIntField(String fieldName, Integer def) {
	Value value = getField(fieldName);
	if (value == null \|\| !value.isInteger()) {
	return def;
	}
	return value.asInteger();
	}

	/**
	* Read a long field of this instance.
	* The field is assumed to be a long type.
	* Returns <code>def</code> if the field value is not an long or could not
	* be read.
	*/
	private Long getLongField(String fieldName, Long def) {
	Value value = getField(fieldName);
	if (value == null \|\| !value.isLong()) {
	return def;
	}
	return value.asLong();
	}

	/**
	* Returns the list of class instance fields for this instance.
	* Includes values of field inherited from the superclass of this instance.
	* The fields are returned in no particular order.
	*
	* @return Iterable over the instance field values.
	*/
	public Iterable<FieldValue> getInstanceFields() {
	return new InstanceFieldIterator(mFields, getClassObj());
	}

	@Override
	Iterable<Reference> getReferences() {
	return new ReferenceIterator();
	}

	@Override public String asString(int maxChars) {
	if (!isInstanceOfClass("java.lang.String")) {
	return null;
	}

	Value value = getField("value");
	if (value == null \|\| !value.isAhatInstance()) {
	return null;
	}

	AhatInstance inst = value.asAhatInstance();
	if (inst.isArrayInstance()) {
	AhatArrayInstance chars = inst.asArrayInstance();
	int numChars = chars.getLength();
	int count = getIntField("count", numChars);
	int offset = getIntField("offset", 0);
	return chars.asMaybeCompressedString(offset, count, maxChars);
	}
	return null;
	}

	@Override public AhatInstance getReferent() {
	if (isInstanceOfClass("java.lang.ref.Reference")) {
	return getRefField("referent");
	}
	return null;
	}

	@Override public String getDexCacheLocation(int maxChars) {
	if (isInstanceOfClass("java.lang.DexCache")) {
	AhatInstance location = getRefField("location");
	if (location != null) {
	return location.asString(maxChars);
	}
	}
	return null;
	}

	@Override public String getBinderProxyInterfaceName() {
	if (isInstanceOfClass("android.os.BinderProxy")) {
	for (AhatInstance inst : getReverseReferences()) {
	String className = inst.getClassName();
	if (className.endsWith("$Stub$Proxy")) {
	Value value = inst.getField("mRemote");
	if (value != null && value.asAhatInstance() == this) {
	return className.substring(0, className.lastIndexOf("$Stub$Proxy"));
	}
	}
	}
	}
	return null;
	}

	@Override public String getBinderTokenDescriptor() {
	String descriptor = getBinderDescriptor();
	if (descriptor == null) {
	return null;
	}

	if (isInstanceOfClass(descriptor + "$Stub")) {
	// This is an instance of an auto-generated interface class, and
	// therefore not a binder token.
	return null;
	}

	return descriptor;
	}

	@Override public String getBinderStubInterfaceName() {
	String descriptor = getBinderDescriptor();
	if (descriptor == null \|\| descriptor.isEmpty()) {
	// Binder interface stubs always have a non-empty descriptor
	return null;
	}

	// We only consider something a binder service if it's an instance of the
	// auto-generated descriptor$Stub class.
	if (isInstanceOfClass(descriptor + "$Stub")) {
	return descriptor;
	}

	return null;
	}

	@Override public AhatInstance getAssociatedBitmapInstance() {
	return getBitmapInfo() == null ? null : this;
	}

	@Override public boolean isClassInstance() {
	return true;
	}

	@Override public AhatClassInstance asClassInstance() {
	return this;
	}

	@Override public String toString() {
	return String.format("%s@%08x", getClassName(), getId());
	}

	/**
	* Returns the descriptor of an android.os.Binder object.
	* If no descriptor is set, returns an empty string.
	* If the object is not an android.os.Binder object, returns null.
	*/
	private String getBinderDescriptor() {
	if (isInstanceOfClass("android.os.Binder")) {
	Value value = getField("mDescriptor");;

	if (value == null) {
	return "";
	} else {
	return value.asAhatInstance().asString();
	}
	} else {
	return null;
	}
	}

	/**
	* Read the given field from the given instance.
	* The field is assumed to be a byte[] field.
	* Returns null if the field value is null, not a byte[] or could not be read.
	*/
	private byte[] getByteArrayField(String fieldName) {
	AhatInstance field = getRefField(fieldName);
	return field == null ? null : field.asByteArray();
	}

	private static class BitmapInfo {
	public final int width;
	public final int height;
	public final byte[] buffer;

	public BitmapInfo(int width, int height, byte[] buffer) {
	this.width = width;
	this.height = height;
	this.buffer = buffer;
	}
	}

	/**
	* Return bitmap info for this object, or null if no appropriate bitmap
	* info is available.
	*/
	private BitmapInfo getBitmapInfo() {
	if (!isInstanceOfClass("android.graphics.Bitmap")) {
	return null;
	}

	Integer width = getIntField("mWidth", null);
	if (width == null) {
	return null;
	}

	Integer height = getIntField("mHeight", null);
	if (height == null) {
	return null;
	}

	byte[] buffer = getByteArrayField("mBuffer");
	if (buffer == null) {
	return null;
	}

	if (buffer.length < 4 * height * width) {
	return null;
	}

	return new BitmapInfo(width, height, buffer);

	}

	@Override public BufferedImage asBitmap() {
	BitmapInfo info = getBitmapInfo();
	if (info == null) {
	return null;
	}

	// Convert the raw data to an image
	// Convert BGRA to ABGR
	int[] abgr = new int[info.height * info.width];
	for (int i = 0; i < abgr.length; i++) {
	abgr[i] = (
	(((int) info.buffer[i * 4 + 3] & 0xFF) << 24)
	+ (((int) info.buffer[i * 4 + 0] & 0xFF) << 16)
	+ (((int) info.buffer[i * 4 + 1] & 0xFF) << 8)
	+ ((int) info.buffer[i * 4 + 2] & 0xFF));
	}

	BufferedImage bitmap = new BufferedImage(
	info.width, info.height, BufferedImage.TYPE_4BYTE_ABGR);
	bitmap.setRGB(0, 0, info.width, info.height, abgr, 0, info.width);
	return bitmap;
	}

	@Override
	RegisteredNativeAllocation asRegisteredNativeAllocation() {
	if (!isInstanceOfClass("sun.misc.Cleaner")) {
	return null;
	}

	Value vthunk = getField("thunk");
	if (vthunk == null \|\| !vthunk.isAhatInstance()) {
	return null;
	}

	AhatClassInstance thunk = vthunk.asAhatInstance().asClassInstance();
	if (thunk == null
	\|\| !thunk.isInstanceOfClass("libcore.util.NativeAllocationRegistry$CleanerThunk")) {
	return null;
	}

	Value vregistry = thunk.getField("this$0");
	if (vregistry == null \|\| !vregistry.isAhatInstance()) {
	return null;
	}

	AhatClassInstance registry = vregistry.asAhatInstance().asClassInstance();
	if (registry == null \|\| !registry.isInstanceOfClass("libcore.util.NativeAllocationRegistry")) {
	return null;
	}

	Value size = registry.getField("size");
	if (!size.isLong()) {
	return null;
	}

	Value referent = getField("referent");
	if (referent == null \|\| !referent.isAhatInstance()) {
	return null;
	}

	RegisteredNativeAllocation rna = new RegisteredNativeAllocation();
	rna.referent = referent.asAhatInstance();
	rna.size = size.asLong();
	return rna;
	}

	private static class InstanceFieldIterator implements Iterable<FieldValue>,
	Iterator<FieldValue> {
	// The complete list of instance field values to iterate over, including
	// superclass field values.
	private Value[] mValues;
	private int mValueIndex;

	// The list of field descriptors specific to the current class in the
	// class hierarchy, not including superclass field descriptors.
	// mFields and mFieldIndex are reset each time we walk up to the next
	// superclass in the call hierarchy.
	private Field[] mFields;
	private int mFieldIndex;
	private AhatClassObj mNextClassObj;

	public InstanceFieldIterator(Value[] values, AhatClassObj classObj) {
	mValues = values;
	mFields = classObj.getInstanceFields();
	mValueIndex = 0;
	mFieldIndex = 0;
	mNextClassObj = classObj.getSuperClassObj();
	}

	@Override
	public boolean hasNext() {
	// If we have reached the end of the fields in the current class,
	// continue walking up the class hierarchy to get superclass fields as
	// well.
	while (mFieldIndex == mFields.length && mNextClassObj != null) {
	mFields = mNextClassObj.getInstanceFields();
	mFieldIndex = 0;
	mNextClassObj = mNextClassObj.getSuperClassObj();
	}
	return mFieldIndex < mFields.length;
	}

	@Override
	public FieldValue next() {
	if (!hasNext()) {
	throw new NoSuchElementException();
	}
	Field field = mFields[mFieldIndex++];
	Value value = mValues[mValueIndex++];
	return new FieldValue(field.name, field.type, value);
	}

	@Override
	public Iterator<FieldValue> iterator() {
	return this;
	}
	}

	/**
	* Returns the reachability type associated with this instance.
	* For example, returns Reachability.WEAK for an instance of
	* java.lang.ref.WeakReference.
	*/
	private Reachability getJavaLangRefType() {
	AhatClassObj cls = getClassObj();
	while (cls != null) {
	switch (cls.getName()) {
	case "java.lang.ref.PhantomReference": return Reachability.PHANTOM;
	case "java.lang.ref.WeakReference": return Reachability.WEAK;
	case "java.lang.ref.FinalizerReference": return Reachability.FINALIZER;
	case "java.lang.ref.Finalizer": return Reachability.FINALIZER;
	case "java.lang.ref.SoftReference": return Reachability.SOFT;
	}
	cls = cls.getSuperClassObj();
	}
	return Reachability.STRONG;
	}

	/**
	* A Reference iterator that iterates over the fields of this instance.
	*/
	private class ReferenceIterator implements Iterable<Reference>,
	Iterator<Reference> {
	private final Iterator<FieldValue> mIter = getInstanceFields().iterator();
	private Reference mNext = null;

	// If we are iterating over a subclass of java.lang.ref.Reference, the
	// 'referent' field doesn't have strong reachability. mJavaLangRefType
	// describes what type of java.lang.ref.Reference subinstance this is.
	private final Reachability mJavaLangRefType = getJavaLangRefType();

	@Override
	public boolean hasNext() {
	while (mNext == null && mIter.hasNext()) {
	FieldValue field = mIter.next();
	if (field.value != null && field.value.isAhatInstance()) {
	Reachability reachability = Reachability.STRONG;
	if (mJavaLangRefType != Reachability.STRONG && "referent".equals(field.name)) {
	reachability = mJavaLangRefType;
	}
	AhatInstance ref = field.value.asAhatInstance();
	mNext = new Reference(AhatClassInstance.this, "." + field.name, ref, reachability);
	}
	}
	return mNext != null;
	}

	@Override
	public Reference next() {
	if (!hasNext()) {
	throw new NoSuchElementException();
	}
	Reference next = mNext;
	mNext = null;
	return next;
	}

	@Override
	public Iterator<Reference> iterator() {
	return this;
	}
	}
	}