数据库类的简介
SQLiteOpenHelper：
管理SQLite的帮助类，提供获取SQLiteDatabase实例的方法，
它会在第一次使用数据库时调用获取实例方法时创建SQLiteDatabase实例，
并且处理数据库版本变化，开发人员在实现ContentProvider时都要实现一个自定义的SQLiteOpenHelper类，
处理数据的创建、升级和降级。SQLiteDatabase：
代表一个打开的SQLite数据库，提供了执行数据库操作的接口方法。如果不需要在进程之间共享数据，
应用程序也可以自行创建这个类的实例来读写SQLite数据库。SQLiteSession：
SQLiteSession负责管理数据库连接和事务的生命周期，
SQLiteSession通过SQLiteConnectionPool获取数据库连接,从而执行具体的数据库操作。SQLiteConnectionPool：
数据库连接池，管理所有打开的数据库连接（Connection）。
所有数据库连接都是通过它来打开，打开后会加入连接池，
在读写数据库时需要从连接池中获取一个数据库连接来使用。SQLiteConnection：
代表了数据库连接，每个Connection封装了一个native层的sqlite3实例，
通过JNI调用SQLite动态库的接口方法操作数据库，Connection要么被Session持有，要么被连接池持有。CursorFactory：
可选的Cursor工厂，可以提供自定义工厂来创建Cursor。DatabaseErrorHandler：
可选的数据库异常处理器（目前仅处理数据库Corruption），如果不提供，将会使用默认的异常处理器。SQLiteDatabaseConfiguration：
数据库配置，应用程序可以创建多个到SQLite数据库的连接，这个类用来保证每个连接的配置都是相同的。SQLiteQuery和SQLiteStatement：
从抽象类SQLiteProgram派生，封装了SQL语句的执行过程，
在执行时自动组装待执行的SQL语句，并调用SQLiteSession来执行数据库操作。
打开数据库连接
通过 getReadableDatabase getWritableDatabase 方法 在使用的时候,
若未打开数据库,则创建数据库连接,
// SQLiteOpenHelper.java
public SQLiteDatabase getReadableDatabase() {
synchronized (this) {
//参数: writable
return getDatabaseLocked(false); //getWritableDatabase 传true
}
}
private SQLiteDatabase getDatabaseLocked(boolean writable) {
if (mDatabase != null) {
...
//check the invailed
return mDatabase;
}
... Isiniting
SQLiteDatabase db = mDatabase;
try {
try {
...
reopenReadWrite()
...
// transfer the path and params and open the db old code
db = SQLiteDatabase.openDatabase(filePath, params); //old code
// new code in wcdb
if (DEBUG_STRICT_READONLY && !writable) {
final String path = mContext.getDatabasePath(mName).getPath();
db = SQLiteDatabase.openDatabase(path, mPassword, mCipher, mFactory,
SQLiteDatabase.OPEN_READONLY, mErrorHandler, connectionPoolSize);
} else {
mNeedMode = true;
mMode = mEnableWriteAheadLogging ? Context.MODE_ENABLE_WRITE_AHEAD_LOGGING : 0;
db = Context.openOrCreateDatabase(mContext, mName, mPassword, mCipher,
mMode, mFactory, mErrorHandler, connectionPoolSize);
}
} catch (SQLException ex) {
// open db will try read-only
params = params.toBuilder().addOpenFlags(SQLiteDatabase.OPEN_READONLY).build();
// not wcdb
// db = SQLiteDatabase.openDatabase(filePath, params);
db = SQLiteDatabase.openDatabase(path, mPassword, mCipher, mFactory,
SQLiteDatabase.OPEN_READONLY, mErrorHandler);
}
}
...
onConfigure(db);
...
// about the version overwrite the method
onCreate
onUpgrade
}
//SQLiteConnectionPool.java
private SQLiteConnectionPool(SQLiteDatabase db, SQLiteDatabaseConfiguration configuration,
int poolSize) {
mDB = new WeakReference<>(db);
//数据库的配置信息
mConfiguration = new SQLiteDatabaseConfiguration(configuration);
//设置最大的数据库链接个数 //未开启wal的版本poolSize默认为1
setMaxConnectionPoolSizeLocked(poolSize);
}
流程：
SQLiteDatabase ->openOrCreateDatabase
-> openDatabase
->open->openInner
->SQLiteConnectionPool.open
->SQLiteConnectionPool.openConnectionLocked
->SQLiteConnection.open
private SQLiteConnection openConnectionLocked(SQLiteDatabaseConfiguration configuration,
boolean primaryConnection) {
//connectionId作为链接id，每次新创建一个数据库链接id自增1
final int connectionId = mNextConnectionId++;
//至此数据库链接被打开 SQLiteConnection.open
//Called by SQLiteConnectionPool only in method SQLiteConnectionPool#openConnectionLocked.
return SQLiteConnection.open(this, configuration,connectionId, primaryConnection); // might throw
}
创建数据库连接场景
- 创建或打开数据库，调用open
- 重新加载数据库配置，调用reconfigure(SQLiteDatabaseConfiguration configuration)
- 创建主链接，调用tryAcquirePrimaryConnectionLocked(int connectionFlags)
- 创建非主链接，调用tryAcquireNonPrimaryConnectionLocked(String sql, int connectionFlags)
SQLiteConnectionPool.png
数据库连接使用
//SQLiteDatabase.java
public long insert(String table, String nullColumnHack, ContentValues values) {
try {
//1.内部封装SQLiteStatement，2.调用statement.executeInsert();
return insertWithOnConflict(table, nullColumnHack, values, CONFLICT_NONE);
} catch (SQLException e) {
return -1;
}
}
//SQLiteStatement.java
public long executeInsert() {
acquireReference();
try {
//通过Session进行数据库操作
return getSession().executeForLastInsertedRowId(
getSql(), getBindArgs(), getConnectionFlags(), null);
} catch (SQLiteDatabaseCorruptException ex) {
checkCorruption(ex);
throw ex;
} finally {
releaseReference();
}
}
// Thread-local for database sessions that belong to this database.
// Each thread has its own database session.
// SQLiteSession的粒度是基于Thread的 每个线程都有一会话，且不可变。
// (Handler内部获取到当前线程的Looper)
private final ThreadLocal mThreadSession = new ThreadLocal() {
@Override
protected SQLiteSession initialValue() {
return createSession();
}
};
SQLiteSession getThreadSession() {
return mThreadSession.get(); // initialValue() throws if database closed
}
SQLiteSession createSession() {
final SQLiteConnectionPool pool;
synchronized (mLock) {
throwIfNotOpenLocked();
pool = mConnectionPoolLocked;
}
return new SQLiteSession(pool);
}
//SQLiteSession.java
/**
* Executes a statement that returns the row id of the last row inserted
* by the statement. Use for INSERT SQL statements.
* @param cancellationSignal A signal to cancel the operation in progress, or null if none.
* @return The row id of the last row that was inserted, or 0 if none.
*/
public long executeForLastInsertedRowId(String sql, Object[] bindArgs, int connectionFlags,
CancellationSignal cancellationSignal) {
//校验sql
if (sql == null) {
throw new IllegalArgumentException("sql must not be null.");
}
//对某些SQL语句（例如“ BEGIN”," COMMIT”和“ ROLLBACK”）执行特殊的重新解释,以确保事务状态不变式为保持。
if (executeSpecial(sql, bindArgs, connectionFlags, cancellationSignal)) {
return 0;
}
//获取数据库链接
acquireConnection(sql, connectionFlags, false, cancellationSignal); // might throw
try {
//使用链接执行数据库操作
return mConnection.executeForLastInsertedRowId(sql, bindArgs,
cancellationSignal); // might throw
} finally {
//释放数据库链接
releaseConnection(); // might throw
}
}
private void acquireConnection(String sql, int connectionFlags,
CancellationSignal cancellationSignal) {
if (mConnection == null) {
assert mConnectionUseCount == 0;
// 从连接池中获取数据库链接
mConnection = mConnectionPool.acquireConnection(sql, connectionFlags,
cancellationSignal); // might throw
mConnectionFlags = connectionFlags;
}
mConnectionUseCount += 1;
}
流程
SQLiteDatabase#insert
->SQLiteDatabase#insertWithOnConflict
->SQLiteStatement#executeInsert (构建sql语句)
->SQLiteSession#executeForLastInsertedRowId(通过Session进行数据库操作)
->SQLiteSession#acquireConnection
->SQLiteConnectionPool#acquireConnection
- 总结:
- Connection从数据库连接池中获取的
- 进行数据库操作是通过Session操作Connection
- 多个线程执行数据库操作会有多个Session(通过ThreadLocal实现)
获取链接
//SQLiteConnectionPool.java
public SQLiteConnection acquireConnection(String sql, int connectionFlags,
CancellationSignal cancellationSignal) {
...
SQLiteConnection connection = waitForConnection(sql, connectionFlags, cancellationSignal);
...
SQLiteTrace callback = mTraceCallback;
if (callback != null) {
long waitTime = SystemClock.uptimeMillis() - startTime;
SQLiteDatabase db = mDB.get();
if (db != null) {
final boolean isPrimary =
(connectionFlags & CONNECTION_FLAG_PRIMARY_CONNECTION_AFFINITY) != 0;
callback.onConnectionObtained(db, sql, waitTime, isPrimary);
}
}
return connection;
}
private SQLiteConnection waitForConnection(String sql, int connectionFlags,
CancellationSignal cancellationSignal) {
//是否需要主链接
final boolean wantPrimaryConnection =
(connectionFlags & CONNECTION_FLAG_PRIMARY_CONNECTION_AFFINITY) != 0;
final ConnectionWaiter waiter;
final int nonce;
synchronized (mLock) {
throwIfClosedLocked();
// Abort if canceled.
//如果取消信号的回调不为空，那么执行回调检测是否需要取消
if (cancellationSignal != null) {
cancellationSignal.throwIfCanceled();
}
// Try to acquire a connection.
//尝试获得一个数据库链接
SQLiteConnection connection = null;
//不需要主链接，尝试获取非主链接
if (!wantPrimaryConnection) {
connection = tryAcquireNonPrimaryConnectionLocked(
sql, connectionFlags); // might throw
}
//获取不到非链接，尝试获取主链接
if (connection == null) {
connection = tryAcquirePrimaryConnectionLocked(connectionFlags); // might throw
}
if (connection != null) {
return connection;
}
// No connections available. Enqueue a waiter in priority order.
//没有可用的连接。按优先级进入等待队列排队
final int priority = getPriority(connectionFlags);
final long startTime = SystemClock.uptimeMillis();
//创建一个等待获取链接的对象
waiter = obtainConnectionWaiterLocked(Thread.currentThread(), startTime,
priority, wantPrimaryConnection, sql, connectionFlags);
ConnectionWaiter predecessor = null;
ConnectionWaiter successor = mConnectionWaiterQueue;
//按照优先级查找插入的位置
while (successor != null) {
if (priority > successor.mPriority) {
waiter.mNext = successor;
break;
}
predecessor = successor;
successor = successor.mNext;
}
//插入等待队列
if (predecessor != null) {
predecessor.mNext = waiter;
} else {
mConnectionWaiterQueue = waiter;
}
nonce = waiter.mNonce;
}
// Set up the cancellation listener.
//设置取消监听器，在等待的过程中如果取消等待那么执行cancelConnectionWaiterLocked
if (cancellationSignal != null) {
cancellationSignal.setOnCancelListener(new CancellationSignal.OnCancelListener() {
@Override
public void onCancel() {
synchronized (mLock) {
if (waiter.mNonce == nonce) {
//从等待队列中删除这个节点数据
//给waiter添加OperationCanceledException异常信息
//唤醒waiter对应线程的阻塞
//调用wakeConnectionWaitersLocked判断队列其他waiter是否状态有更新
cancelConnectionWaiterLocked(waiter);
}
}
}
});
}
try {
// Park the thread until a connection is assigned or the pool is closed.
// Rethrow an exception from the wait, if we got one.
//驻留线程，直到分配了连接或关闭了池。
//如果有异常，则从等待中抛出异常。
long busyTimeoutMillis = CONNECTION_POOL_BUSY_MILLIS;
long nextBusyTimeoutTime = waiter.mStartTime + busyTimeoutMillis;
for (;;) {
// Detect and recover from connection leaks.
//是否需要从泄漏中进行恢复，之前被调用onConnectionLeaked //mConnectionLeaked.set(true);
// 获取连接的策略: 定期检查此标志，以便它可以从泄漏的连接中恢复并唤醒
if (mConnectionLeaked.compareAndSet(true, false)) {
synchronized (mLock) {
//为等待数据库链接队列进行链接赋值
wakeConnectionWaitersLocked();
}
}
// how to stop?
// Wait to be unparked (may already have happened), a timeout, or interruption.
//阻塞busyTimeoutMillis毫秒，或者中间被执行LockSupport.unpark (释放许可)
//被执行cancelConnectionWaiterLocked进行取消
//或者被执行wakeConnectionWaitersLocked进行链接分配
LockSupport.parkNanos(this, busyTimeoutMillis * 1000000L);
// Clear the interrupted flag, just in case.
Thread.interrupted();//重置当前线程的中断状态
// Check whether we are done waiting yet.
//检查我们是否已经完成等待。
synchronized (mLock) {
throwIfClosedLocked();//如果数据库关闭，那么抛出异常
final SQLiteConnection connection = waiter.mAssignedConnection;
final RuntimeException ex = waiter.mException;
//如果已经分配到链接，或者异常
if (connection != null || ex != null) {
recycleConnectionWaiterLocked(waiter);//回收waiter
if (connection != null) {//返回分配链接
return connection;
}
throw ex; // rethrow!重新抛出异常
}
final long now = SystemClock.uptimeMillis();
if (now < nextBusyTimeoutTime) {
//parkNanos阻塞时间不够busyTimeoutMillis毫秒，被执行LockSupport.unpark
busyTimeoutMillis = now - nextBusyTimeoutTime;
} else {
busyInfo = gatherConnectionPoolBusyInfoLocked();
//重置下次阻塞时间
busyTimeoutMillis = CONNECTION_POOL_BUSY_MILLIS;
nextBusyTimeoutTime = now + busyTimeoutMillis;
}
if (busyInfo != null) {
long waitMillis = now - waiter.mStartTime;
//输出日志
logConnectionPoolBusy(busyInfo, waitMillis, connectionFlags);
SQLiteDatabase db = mDB.get();
SQLiteTrace callback = mTraceCallback;
if (db != null && callback != null) {
// SQLiteTrace 性能监控的回调
callback.onConnectionPoolBusy(db, sql, waitMillis, wantPrimaryConnection,
busyInfo.activeSql, busyInfo.activeTransactions);
}
}
}
}
} finally {
// Remove the cancellation listener.
//有异常，或者获取到分配的链接 那么解绑"取消信号回调"信息
if (cancellationSignal != null) {
cancellationSignal.setOnCancelListener(null);
}
}
}
acquireDBConnection.png
LockSupport.parkNanos 循环判断是否获得了数据库链接,未获取到则继续睡眠,
直到这次操作被取消或者获得数据库链接。LockSupport.park() 的实现原理是通过二元信号量实现阻塞，
这个信号量最多只能加到1。可以理解为获取释放许可证的场景。
unpark()方法会释放一个许可证，park()方法则是获取许可证，如果当前没有许可证，
则进入休眠状态，知道许可证被释放了才被唤醒。无论执行多少次unpark()方法，也最多只会有一个许可证。
主链接的获取
//SQLiteConnectionPool.java
@GuardedBy("mLock")
private SQLiteConnection tryAcquirePrimaryConnectionLocked(int connectionFlags) {
// If the primary connection is available, acquire it now.
SQLiteConnection connection = mAvailablePrimaryConnection;
if (connection != null) {
mAvailablePrimaryConnection = null;
finishAcquireConnectionLocked(connection, connectionFlags); // might throw
return connection;
}
// Make sure that the primary connection actually exists and has just been acquired.
for (SQLiteConnection acquiredConnection : mAcquiredConnections.keySet()) {
if (acquiredConnection.isPrimaryConnection()) {
return null;
}
}
// Uhoh. No primary connection! Either this is the first time we asked
// for it, or maybe it leaked?
//第一次创建数据库主链接，或者主链接被回收
connection = openConnectionLocked(mConfiguration,
true /*primaryConnection*/); // might throw
finishAcquireConnectionLocked(connection, connectionFlags); // might throw
return connection;
}
- 总结: 主数据库链接只一个，如果被占用那么需要等待，如果没有那么就需要创建。
获取非主链接
//SQLiteConnectionPool.java
private SQLiteConnection tryAcquireNonPrimaryConnectionLocked(
String sql, int connectionFlags) {
// Try to acquire the next connection in the queue.
SQLiteConnection connection;
//尝试获取队列中的下一个连接。
final int availableCount = mAvailableNonPrimaryConnections.size();
if (availableCount > 1 && sql != null) {
// If we have a choice, then prefer a connection that has the
// prepared statement in its cache.
//检查是否可以在其缓存中选择包含prepare语句的连接。
for (int i = 0; i < availableCount; i++) {
connection = mAvailableNonPrimaryConnections.get(i);
if (connection.isPreparedStatementInCache(sql)) {
// 从非主链接集合中移除
mAvailableNonPrimaryConnections.remove(i);
finishAcquireConnectionLocked(connection, connectionFlags); // might throw
return connection;
}
}
}
if (availableCount > 0) {
// Otherwise, just grab the last one.//next one 抓取非主链接集合中的最后一个
connection = mAvailableNonPrimaryConnections.remove(availableCount - 1);
finishAcquireConnectionLocked(connection, connectionFlags); // might throw
return connection;
}
//如果没有可以的非主链接，则扩展数据库连接池
// Expand the pool if needed.
int openConnections = mAcquiredConnections.size();
if (mAvailablePrimaryConnection != null) {
openConnections += 1;
}
//如果数据库连接池已经达到上限，返回null
if (openConnections >= mMaxConnectionPoolSize) {
return null;
}
//否则创建新的非主链接
connection = openConnectionLocked(mConfiguration,
false /*primaryConnection*/); // might throw
finishAcquireConnectionLocked(connection, connectionFlags); // might throw
return connection;
}
- 非主数据库链接数量的多少受限于数据库连接池的大小。
数据库链接释放
//SQLiteConnectionPool.java
// Weak references to all acquired connections. The associated value
// indicates whether the connection must be reconfigured before being
// returned to the available connection list or discarded.
// For example, the prepared statement cache size may have changed and
// need to be updated in preparation for the next client.
private final WeakHashMap mAcquiredConnections
= new WeakHashMap<>();
//释放数据库链接,使其返回连接池
public void releaseConnection(SQLiteConnection connection) {
synchronized (mLock) {
//idle事件处理connectionReleased
if (mIdleConnectionHandler != null) {
mIdleConnectionHandler.connectionReleased(connection);
}
//获取这个链接的状态 by invoke recycleConnectionLocked
//NORMAL,正常返回连接池
//RECONFIGURE,必须先重新配置连接，然后才能返回。
//DISCARD,连接必须关闭并丢弃。
AcquiredConnectionStatus status = mAcquiredConnections.remove(connection);
if (status == null) {
throw new IllegalStateException("Cannot perform this operation "
+ "because the specified connection was not acquired "
+ "from this pool or has already been released.");
}
//检测是否已经关闭连接池
if (!mIsOpen) {
closeConnectionAndLogExceptionsLocked(connection);
} else if (connection.isPrimaryConnection()) {//如果是主链接
//判断这个数据库链接是否需要回收
if (recycleConnectionLocked(connection, status)) {
assert mAvailablePrimaryConnection == null;
mAvailablePrimaryConnection = connection;//标识主链接可用，被占用的时候为null
}
//判断队列其他waiter是否状态有更新
wakeConnectionWaitersLocked();
} else if (mAvailableNonPrimaryConnections.size() >= mMaxConnectionPoolSize - 1) {
//可用的非主链接数+主链接>=最大链接数的时 关闭这个链接
closeConnectionAndLogExceptionsLocked(connection);
} else {
//判断这个数据库链接是否需要回收 不回收则添加
if (recycleConnectionLocked(connection, status)) {
//将这个链接添加到非主链接容器中
mAvailableNonPrimaryConnections.add(connection);
}
//判断队列其他waiter是否状态有更新
wakeConnectionWaitersLocked();
}
}
}
//唤醒waiter对应的线程 取消已获取许可的park
//将异常赋值给waiter进行抛出
@GuardedBy("mLock")
private void wakeConnectionWaitersLocked() {
// 释放等待
// Unpark all waiters that have requests that we can fulfill.
// This method is designed to not throw runtime exceptions, although we might send
// a waiter an exception for it to rethrow.
ConnectionWaiter predecessor = null;
//链表的head point
ConnectionWaiter waiter = mConnectionWaiterQueue;
boolean primaryConnectionNotAvailable = false;
boolean nonPrimaryConnectionNotAvailable = false;
while (waiter != null) {
boolean unpark = false;
//是否关闭了数据库，如果关闭了那么唤醒所有waiter的线程
if (!mIsOpen) {
unpark = true;
} else {
try {
SQLiteConnection connection = null;
//如果该waiter需要非主链接，而且现在有可用的非主链接
if (!waiter.mWantPrimaryConnection && !nonPrimaryConnectionNotAvailable) {
//获取非主链接
connection = tryAcquireNonPrimaryConnectionLocked(
waiter.mSql, waiter.mConnectionFlags); // might throw
//获取为空，标识现在没有可用的非主链接
if (connection == null) {
nonPrimaryConnectionNotAvailable = true;
}
}
//主链接可以用
if (connection == null && !primaryConnectionNotAvailable) {
//尝试获取主链接
connection = tryAcquirePrimaryConnectionLocked(
waiter.mConnectionFlags); // might throw
//获取为空，标识现在主链接不可用
if (connection == null) {
primaryConnectionNotAvailable = true;
}
}
//获取到了数据库链接
if (connection != null) {
waiter.mAssignedConnection = connection;//改waiter赋值链接
unpark = true;//唤醒该waiter的对应线程
} else if (nonPrimaryConnectionNotAvailable && primaryConnectionNotAvailable) {
// There are no connections available and the pool is still open.
// We cannot fulfill any more connection requests, so stop here.
//连接池仍然可用，但是没有可用的链接,无法对其他的waiter状态进行更新则直接返回
break;
}
} catch (RuntimeException ex) {
// Let the waiter handle the exception from acquiring a connection.
waiter.mException = ex;
unpark = true;
}
}
final ConnectionWaiter successor = waiter.mNext;
//如果需要唤醒，那么从链表中删除这个waiter，并进行对应线程唤醒操作
if (unpark) {
if (predecessor != null) {
predecessor.mNext = successor;
} else {
mConnectionWaiterQueue = successor;
}
waiter.mNext = null;
LockSupport.unpark(waiter.mThread);
} else {
predecessor = waiter;
}
waiter = successor;
}
}
- 链接的释放有时候是为了回收，有时候为了重用。重用的时候还需要唤醒等待链接队列中获得这个链接的waiter 。
数据库链接池的关闭
数据库的关闭
//SQLiteClosable.java，它是SQLiteDatabase的父类
//释放引用的对象，直到所有的引用都被释放了那么关闭数据库
public void close() {
releaseReference();
}
public void releaseReference() {
boolean refCountIsZero;
synchronized (this) {
refCountIsZero = --mReferenceCount == 0;
}
//引用计数递减
if (refCountIsZero) {
onAllReferencesReleased();
}
}
//SQLiteDatabase.java
@Override
protected void onAllReferencesReleased() {
dispose(false);
}
private void dispose(boolean finalized) {
final SQLiteConnectionPool pool;
synchronized (mLock) {
pool = mConnectionPoolLocked;
//连接池置空，无法进行新操作
mConnectionPoolLocked = null;
}
if (!finalized) {
synchronized (sActiveDatabases) {
//删除当前数据库的引用
sActiveDatabases.remove(this);
}
// 触发连接池的关闭
if (pool != null) {
pool.close(); //SQLiteConnectionPool.close()
}
}
}
- 数据库连接池的关闭是由数据库关闭触发的。
数据库链接池的关闭
//关闭数据库连接池，停止接受新的数据库链接的请求。
//链接池中的可用链接立即被关闭，其他正在使用的链接被归还到数据的时候关闭
public void close() {
dispose(false);
}
private void dispose(boolean finalized) {
if (!finalized) {
// Close all connections. We don't need (or want) to do this
// when finalized because we don't know what state the connections
// themselves will be in. The finalizer is really just here for CloseGuard.
// The connections will take care of themselves when their own finalizers run.
synchronized (mLock) {
throwIfClosedLocked();//检测是否已经被关闭
mIsOpen = false;//标识数据库连接池关闭
//关闭数据库连接池中目前可用的链接（可用的主链接与非主链接集合）
closeAvailableConnectionsAndLogExceptionsLocked(); //SQLiteConnection.close()
final int pendingCount = mAcquiredConnections.size();
//仍然有链接正在使用中
if (pendingCount != 0) {
Log.i(TAG, "The connection pool for " + mConfiguration.label
+ " has been closed but there are still "
+ pendingCount + " connections in use. They will be closed "
+ "as they are released back to the pool.");
}
//队列其他waiter是否状态有更新
wakeConnectionWaitersLocked();
}
}
}
数据库链接的关闭
//SQLiteConnectionPool.java
// Can't throw.
@GuardedBy("mLock")
private void closeAvailableConnectionsAndLogExceptionsLocked() {
//关闭可用的非主链接
closeAvailableNonPrimaryConnectionsAndLogExceptionsLocked();
//关闭主链接
if (mAvailablePrimaryConnection != null) {
closeConnectionAndLogExceptionsLocked(mAvailablePrimaryConnection);
mAvailablePrimaryConnection = null;
}
}
// Can't throw.
@GuardedBy("mLock")
private void closeAvailableNonPrimaryConnectionsAndLogExceptionsLocked() {
//循环遍历可用的非主链接，关闭链接
final int count = mAvailableNonPrimaryConnections.size();
for (int i = 0; i < count; i++) {
closeConnectionAndLogExceptionsLocked(mAvailableNonPrimaryConnections.get(i));
}
mAvailableNonPrimaryConnections.clear();
}
// Can't throw.
@GuardedBy("mLock")
private void closeConnectionAndLogExceptionsLocked(SQLiteConnection connection) {
try {
connection.close(); // might throw
if (mIdleConnectionHandler != null) {
mIdleConnectionHandler.connectionClosed(connection);
}
} catch (RuntimeException ex) {
Log.e(TAG, "Failed to close connection, its fate is now in the hands "
+ "of the merciful GC: " + connection, ex);
}
}
- 数据库关闭的时候引用次数自减，若引用次数归零则真正执行关闭数据库；
- 数据库关闭清除引用后,进行数据库连接池的关闭；
- 关闭所有的空闲链接，使用中的连接回归连接池后被关闭；
并发问题的解决
历史实现： SQLite的原子操作实现
默认方法是 rollback journal。
当对 DB 进行写操作的时候，SQLite 首先将准备要修改部分的原始内容（以 Page 为单位）拷贝到“回滚日志”中，用于后续可能的 Rollback操作以及 Crash、断电等意外造成写操作中断时恢复 DB 的原始状态，回滚日志存放于名为“DB文件名-journal”的独立文件中（以下简称“-journal”）。对原始内容做备份后，才能写入修改后的内容到 DB 主文件中，
当写入操作完成，用户提交事务后，SQLite 清空 -journal 的内容，至此完成一个完整的写事务。
rollback_journal.png
Rollback 模式中，每次拷贝原始内容或写入新内容后，都需要确保之前写入的数据真正写入到磁盘，而不是缓存在操作系统中，这需要发起一次 fsync 操作，通知并等待操作系统将缓存真正写入磁盘，这个过程十分耗时。
除了耗时的 fsync 操作，写入 -journal 以及 DB 主文件的时候，是需要独占整个 DB 的，否则别的线程/进程可能读取到写到一半的内容。这样的设计使得写操作与读操作是互斥的，并发性很差。
WAL模式和异步Checkpoint
- WCDB 开启了 WAL 模式
- 针对WAL做了改进,使用异步Checkpoint
WAL 模式+则改变了上述流程，写操作不直接写入 DB 主文件，而是写到“DB文件名-wal”文件的末尾，并且通过 -shm 共享内存文件来实现 -wal 内容的索引。读操作时，将结合 DB 主文件以及 -wal 的内容返回结果。由于读操作只读取 DB 主文件和 -wal 前面非正在写入的部分，不需要读取写操作正在写到一半的内容，WAL 模式下读与写操作的并发由此实现。
WAL 写操作除了上面的流程，还增加了一步：Checkpoint，即将 -wal 的内容与合并到 DB 主文件。 由于写操作将内容临时写到 -wal 文件，-wal 文件会不断增大且拖慢读操作，因此需要定期进行 Checkpoint 操作将 -wal 文件保持在合理的大小。Checkpoint 操作比较耗时且会阻塞读操作，但由于时效性要求较低，遇到堵塞可以暂时放弃Checkpoint 操作. 继续 DB 读写操作，不至于太过影响读写性能。SQLite 官方默认的 Checkpoint 阈值是 1000 page，即当 -wal 文件达到 1000 page 大小时，写操作的线程在完成写操作后,再同步进行 Checkpoint 操作；Android Framework 的 Checkpoint 阈值是 100 page。
wal.png
基于 WAL 的基本工作方式，两个优化点：
- 写入 -wal 文件时不进行 fsync 操作，因为 -wal 文件损坏只影响新写入的未 Checkpoint 部分数据而非整个数据库损坏，影响相对小
- 将需要进行fsync的Checkpoint 操作放到独立线程执行，让写操作能尽快返回
异步Checkpoint的基本思路，减少和转移耗时较多而且性能不稳定的 fsync 操作，增强写操作性能和减少突然卡顿的可能性，同时不增加 DB 损坏率。
// SQLiteConnectionPool.java
SQLiteCheckpointListener getCheckpointListener() {
return mCheckpointListener;
}
void setCheckpointListener(SQLiteCheckpointListener listener) {
SQLiteDatabase db = mDB.get();
if (mCheckpointListener != null)
mCheckpointListener.onDetach(db);
mCheckpointListener = listener;
if (mCheckpointListener != null)
mCheckpointListener.onAttach(db);
}
// 调用通过jni
/*package*/ void notifyCheckpoint(String dbName, int pages) {
SQLiteDatabase db = mDB.get();
SQLiteCheckpointListener walHook = mCheckpointListener;
if (walHook == null || db == null)
return;
walHook.onWALCommit(db, dbName, pages);
}
public interface SQLiteCheckpointListener {
void onAttach(SQLiteDatabase db);
//Called immediately when a WAL transaction has been committed
void onWALCommit(SQLiteDatabase db, String dbName, int pages);
void onDetach(SQLiteDatabase db);
}
// 实现类: SQLiteAsyncCheckpointer
public SQLiteAsyncCheckpointer(Looper looper, int threshold, int blockingThreshold) {
mLooper = looper;
mThreshold = threshold;
mBlockingThreshold = blockingThreshold;
mPendingCheckpoints = new HashSet<>();
}
@Override
public void onAttach(SQLiteDatabase db) {
if (mLooper == null) {
mLooper = acquireDefaultLooper();
mUseDefaultLooper = true;
}
mHandler = new Handler(mLooper, this);
mLastSyncMode = db.getSynchronousMode();
db.setSynchronousMode(SQLiteDatabase.SYNCHRONOUS_NORMAL);
}
/**
* shmily add note
* 此处要关注WAL的提交策略
*/
@Override
public void onWALCommit(SQLiteDatabase db, String dbName, int pages) {
if (pages < mThreshold)
return;
boolean blockWriting = pages >= mBlockingThreshold;
Pair p = new Pair<>(db, dbName);
boolean newTask;
synchronized (mPendingCheckpoints) {
newTask = mPendingCheckpoints.add(p);
}
if (!newTask)
return;
/**
* shmily add note
* acquireReference 与 releaseReference 成对调用
* 用于引用计数 引用则次数+1 释放则次数-1
* 在sendMessage之前acquireReference
* handMessage处理完成后释放
*/
db.acquireReference();
Message msg = mHandler.obtainMessage(0, blockWriting ? 1 : 0, 0, p);
mHandler.sendMessage(msg);
}
@Override
public void onDetach(SQLiteDatabase db) {
// 恢复SynchronousMode
db.setSynchronousMode(mLastSyncMode);
mHandler = null;
if (mUseDefaultLooper) {
mLooper = null;
releaseDefaultLooper();
mUseDefaultLooper = false;
}
}
@Override
@SuppressWarnings("unchecked")
public boolean handleMessage(Message msg) {
Pair p = (Pair) msg.obj;
// Pair 的方便之处
SQLiteDatabase db = p.first;
String dbName = p.second;
boolean blockWriting = msg.arg1 != 0;
try {
long time = SystemClock.uptimeMillis();
Pair result = db.walCheckpoint(dbName, blockWriting);
int walPages = result.first;
int checkpointedPages = result.second;
time = SystemClock.uptimeMillis() - time;
onCheckpointResult(db, walPages, checkpointedPages, time);
} finally {
db.releaseReference();
}
synchronized (mPendingCheckpoints) {
if (!mPendingCheckpoints.remove(p))
throw new AssertionError("mPendingCheckpoints.remove(p)");
}
return true;
}
protected void onCheckpointResult(SQLiteDatabase db, int walPages, int checkpointedPages,
long time) {
// implemented by the derived class
}
private static Looper acquireDefaultLooper() {
synchronized (gDefaultThreadLock) {
if (gDefaultThreadRefCount++ == 0) {
// Initialize default handler thread.
if (gDefaultThread != null)
throw new AssertionError("gDefaultThread == null");
gDefaultThread = new HandlerThread("WCDB.AsyncCheckpointer", 4);
gDefaultThread.start();
}
return gDefaultThread.getLooper();
}
}
private static void releaseDefaultLooper() {
synchronized (gDefaultThreadLock) {
if (--gDefaultThreadRefCount <= 0) {
if (gDefaultThreadRefCount < 0)
throw new AssertionError("gDefaultThreadRefCount == 0");
gDefaultThread.quit();
gDefaultThread = null;
}
}
}