View Javadoc

1   /**
2    *
3    * Licensed to the Apache Software Foundation (ASF) under one
4    * or more contributor license agreements.  See the NOTICE file
5    * distributed with this work for additional information
6    * regarding copyright ownership.  The ASF licenses this file
7    * to you under the Apache License, Version 2.0 (the
8    * "License"); you may not use this file except in compliance
9    * with the License.  You may obtain a copy of the License at
10   *
11   *     http://www.apache.org/licenses/LICENSE-2.0
12   *
13   * Unless required by applicable law or agreed to in writing, software
14   * distributed under the License is distributed on an "AS IS" BASIS,
15   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
16   * See the License for the specific language governing permissions and
17   * limitations under the License.
18   */
19  
20  package org.apache.hadoop.hbase.util;
21  
22  import java.util.concurrent.atomic.AtomicLong;
23  
24  import org.apache.hadoop.classification.InterfaceAudience;
25  
26  /**
27   * A simple barrier that can be used by classes that need to wait for some operations to
28   * finish before stopping/closing/etc. forever.
29   */
30  @InterfaceAudience.Private
31  public class DrainBarrier {
32    /**
33     * Contains the number of outstanding operations, as well as flags.
34     * Initially, the number of operations is 1. Each beginOp increments, and endOp decrements it.
35     * beginOp does not proceed when it sees the draining flag. When stop is called, it atomically
36     * decrements the number of operations (the initial 1) and sets the draining flag. If stop did
37     * the decrement to zero, that means there are no more operations outstanding, so stop is done.
38     * Otherwise, stop blocks, and the endOp that decrements the count to 0 unblocks it.
39     */
40    private final AtomicLong valueAndFlags = new AtomicLong(inc(0));
41    private final static long DRAINING_FLAG = 0x1;
42    private final static int FLAG_BIT_COUNT = 1;
43  
44    /**
45     * Tries to start an operation.
46     * @return false iff the stop is in progress, and the operation cannot be started.
47     */
48    public boolean beginOp() {
49      long oldValAndFlags;
50      do {
51        oldValAndFlags = valueAndFlags.get();
52        if (isDraining(oldValAndFlags)) return false;
53      } while (!valueAndFlags.compareAndSet(oldValAndFlags, inc(oldValAndFlags)));
54      return true;
55    }
56  
57    /**
58     * Ends the operation. Unblocks the blocked caller of stop, if necessary.
59     */
60    @edu.umd.cs.findbugs.annotations.SuppressWarnings(value="NN_NAKED_NOTIFY",
61        justification="First, we do change the state before notify, 2nd, it doesn't even matter")
62    public void endOp() {
63      long oldValAndFlags;
64      do {
65        oldValAndFlags = valueAndFlags.get();
66        long unacceptableCount = isDraining(oldValAndFlags) ? 0 : 1;
67        if (getValue(oldValAndFlags) == unacceptableCount) {
68          throw new AssertionError("endOp called without corresponding beginOp call ("
69            + "the current count is " + unacceptableCount + ")");
70        }
71      } while (!valueAndFlags.compareAndSet(oldValAndFlags, dec(oldValAndFlags)));
72      if (getValue(oldValAndFlags) == 1) {
73        synchronized (this) { this.notifyAll(); }
74      }
75    }
76  
77    /**
78     * Blocks new operations from starting, waits for the current ones to drain.
79     * If someone already called it, returns immediately, which is currently unavoidable as
80     * most of the users stop and close things right and left, and hope for the best.
81     * stopAndWaitForOpsOnce asserts instead.
82     * @throws InterruptedException the wait for operations has been interrupted.
83     */
84    public void stopAndDrainOps() throws InterruptedException {
85      stopAndDrainOps(true);
86    }
87  
88    /**
89     * Blocks new operations from starting, waits for the current ones to drain.
90     * Can only be called once.
91     * @throws InterruptedException the wait for operations has been interrupted.
92     */
93    public void stopAndDrainOpsOnce() throws InterruptedException {
94      stopAndDrainOps(false);
95    }
96  
97    /**
98     * @param ignoreRepeatedCalls If this is true and somebody already called stop, this method
99     *                            will return immediately if true; if this is false and somebody
100    *                            already called stop, it will assert.
101    */
102   // Justification for warnings - wait is not unconditional, and contrary to what WA_NOT_IN_LOOP
103   // description says we are not waiting on multiple conditions.
104   @edu.umd.cs.findbugs.annotations.SuppressWarnings({"UW_UNCOND_WAIT", "WA_NOT_IN_LOOP"})
105   private void stopAndDrainOps(boolean ignoreRepeatedCalls) throws InterruptedException {
106     long oldValAndFlags;
107     do {
108       oldValAndFlags = valueAndFlags.get();
109       if (isDraining(oldValAndFlags)) {
110         if (ignoreRepeatedCalls) return;
111         throw new AssertionError("stopAndWaitForOpsOnce called more than once");
112       }
113     } while (!valueAndFlags.compareAndSet(oldValAndFlags, dec(oldValAndFlags) | DRAINING_FLAG));
114     if (getValue(oldValAndFlags) == 1) return; // There were no operations outstanding.
115     synchronized (this) { this.wait(); }
116   }
117 
118   // Helper methods.
119   private static final boolean isDraining(long valueAndFlags) {
120     return (valueAndFlags & DRAINING_FLAG) == DRAINING_FLAG;
121   }
122 
123   private static final long getValue(long valueAndFlags) {
124     return valueAndFlags >> FLAG_BIT_COUNT;
125   }
126 
127   private static final long inc(long valueAndFlags) {
128     return valueAndFlags + (1 << FLAG_BIT_COUNT); // Not checking for overflow.
129   }
130 
131   private static final long dec(long valueAndFlags) {
132     return valueAndFlags - (1 << FLAG_BIT_COUNT); // Negative overflow checked outside.
133   }
134 }