1/*
2 * Copyright (C) 2016 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16package android.arch.persistence.room;
17
18import android.arch.persistence.db.SupportSQLiteStatement;
19import android.support.annotation.RestrictTo;
20
21import java.util.concurrent.atomic.AtomicBoolean;
22
23/**
24 * Represents a prepared SQLite state that can be re-used multiple times.
25 * <p>
26 * This class is used by generated code. After it is used, {@code release} must be called so that
27 * it can be used by other threads.
28 * <p>
29 * To avoid re-entry even within the same thread, this class allows only 1 time access to the shared
30 * statement until it is released.
31 *
32 * @hide
33 */
34@SuppressWarnings({"WeakerAccess", "unused"})
35@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
36public abstract class SharedSQLiteStatement {
37    private final AtomicBoolean mLock = new AtomicBoolean(false);
38
39    private final RoomDatabase mDatabase;
40    private volatile SupportSQLiteStatement mStmt;
41
42    /**
43     * Creates an SQLite prepared statement that can be re-used across threads. If it is in use,
44     * it automatically creates a new one.
45     *
46     * @param database The database to create the statement in.
47     */
48    public SharedSQLiteStatement(RoomDatabase database) {
49        mDatabase = database;
50    }
51
52    /**
53     * Create the query.
54     *
55     * @return The SQL query to prepare.
56     */
57    protected abstract String createQuery();
58
59    protected void assertNotMainThread() {
60        mDatabase.assertNotMainThread();
61    }
62
63    private SupportSQLiteStatement createNewStatement() {
64        String query = createQuery();
65        return mDatabase.compileStatement(query);
66    }
67
68    private SupportSQLiteStatement getStmt(boolean canUseCached) {
69        final SupportSQLiteStatement stmt;
70        if (canUseCached) {
71            if (mStmt == null) {
72                mStmt = createNewStatement();
73            }
74            stmt = mStmt;
75        } else {
76            // it is in use, create a one off statement
77            stmt = createNewStatement();
78        }
79        return stmt;
80    }
81
82    /**
83     * Call this to get the statement. Must call {@link #release(SupportSQLiteStatement)} once done.
84     */
85    public SupportSQLiteStatement acquire() {
86        assertNotMainThread();
87        return getStmt(mLock.compareAndSet(false, true));
88    }
89
90    /**
91     * Must call this when statement will not be used anymore.
92     *
93     * @param statement The statement that was returned from acquire.
94     */
95    public void release(SupportSQLiteStatement statement) {
96        if (statement == mStmt) {
97            mLock.set(false);
98        }
99    }
100}
101