Java example source code file (MarkBits.java)

This example Java source code file (MarkBits.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

address, bitmap, markbits, memregion, oophandle

The MarkBits.java Java example source code

/*
 * Copyright (c) 2001, 2003, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

package sun.jvm.hotspot.utilities;

import sun.jvm.hotspot.debugger.*;
import sun.jvm.hotspot.gc_interface.*;
import sun.jvm.hotspot.memory.*;
import sun.jvm.hotspot.oops.*;
import sun.jvm.hotspot.runtime.*;

/** Helper class which covers the reserved area of the heap with an
    (object-external) set of mark bits, used for GC-like scans through
    the heap like liveness analysis. */

public class MarkBits {
  public MarkBits(CollectedHeap heap) {
    MemRegion reserved = heap.reservedRegion();
    // Must cover "reserved" with one bit for each OopHandle
    start = reserved.start();
    end   = reserved.end();
    long numOopHandles = end.minus(start) / VM.getVM().getOopSize();
    // FIXME: will have trouble with larger heap sizes
    bits = new BitMap((int) numOopHandles);
  }

  public void clear() {
    bits.clear();
  }

  /** Returns true if a mark was newly placed for the given Oop, or
      false if the Oop was already marked. If the Oop happens to lie
      outside the heap (should not happen), prints a warning and
      returns false. */
  public boolean mark(Oop obj) {
    if (obj == null) {
      System.err.println("MarkBits: WARNING: null object, ignoring");
      return false;
    }

    OopHandle handle = obj.getHandle();
    // FIXME: will have trouble with larger heap sizes
    long idx = handle.minus(start) / VM.getVM().getOopSize();
    if ((idx < 0) || (idx >= bits.size())) {
      System.err.println("MarkBits: WARNING: object " + handle + " outside of heap, ignoring");
      return false;
    }
    int intIdx = (int) idx;
    if (bits.at(intIdx)) {
      return false; // already marked
    }
    bits.atPut(intIdx, true);
    return true;
  }

  /** Forces clearing of a given mark bit. */
  public void clear(Oop obj) {
    OopHandle handle = obj.getHandle();
    // FIXME: will have trouble with larger heap sizes
    long idx = handle.minus(start) / VM.getVM().getOopSize();
    if ((idx < 0) || (idx >= bits.size())) {
      System.err.println("MarkBits: WARNING: object " + handle + " outside of heap, ignoring");
      return;
    }
    int intIdx = (int) idx;
    bits.atPut(intIdx, false);
  }

  private BitMap  bits;
  private Address start;
  private Address end;
}