SQLiteDatabase这个类，大家都不陌生。其中：
- [
int delete(String table, String whereClause, String[] whereArgs)](https://developer.android.com/reference/android/database/sqlite/SQLiteDatabase.html#delete(java.lang.String, java.lang.String, java.lang.String[])) - [
Cursor query(String table, String[] columns, String whereClause, String[] whereArgs, String groupBy, String having, String orderBy, String limit)](https://developer.android.com/reference/android/database/sqlite/SQLiteDatabase.html#query(java.lang.String, java.lang.String[], java.lang.String, java.lang.String[], java.lang.String, java.lang.String, java.lang.String, java.lang.String)) - [
int update(String table, ContentValues values, String whereClause, String[] whereArgs)](https://developer.android.com/reference/android/database/sqlite/SQLiteDatabase.html#update(java.lang.String, android.content.ContentValues, java.lang.String, java.lang.String[]))
增删改查，有三个接口需要进行条件限定。限定条件通过参数String whereClause, String[] whereArgs来指定。
whereClause的值形如_id = ? AND condition1 >= ? OR condition2 != ?，其中的?用于参数绑定，按顺序，填入whereArgs数组内。
但说实话，使用这种方式，需要先将限定部分的SQL语句写出来，将限定的参数替换为?，然后记住次序，填入数组内。一次还好，写多了挺烦人的，如果是修改的话，还需要仔细确保SQL语句书写正确，确保修改不会弄错参数顺序。
为了方便，有的同学们就直接放弃了whereClause和whereArgs这种搭配，直接传入完整的SQL限定字符串作为whereClause参数的值，在whereArgs参数传入了null。
这种用法同样能达成我们的需求，为什么SDK要搞得这么复杂呢？答：一切都是为了性能。
让我们来看下源码里是怎么处理的：
以int delete(String table, String whereClause, String[] whereArgs)这个方法为切入点，相关实现在SQLiteDatabase.java里。
/**
* Convenience method for deleting rows in the database.
*
* @param table the table to delete from
* @param whereClause the optional WHERE clause to apply when deleting.
* Passing null will delete all rows.
* @param whereArgs You may include ?s in the where clause, which
* will be replaced by the values from whereArgs. The values
* will be bound as Strings.
* @return the number of rows affected if a whereClause is passed in, 0
* otherwise. To remove all rows and get a count pass "1" as the
* whereClause.
*/
public int delete(String table, String whereClause, String[] whereArgs) {
acquireReference();
try {
// 组装成完整的SQL语句，实例化SQLiteStatement
SQLiteStatement statement = new SQLiteStatement(this, "DELETE FROM " + table +
(!TextUtils.isEmpty(whereClause) ? " WHERE " + whereClause : ""), whereArgs);
try {
// 执行SQL语句
return statement.executeUpdateDelete();
} finally {
statement.close();
}
} finally {
releaseReference();
}
}
源码里并没做什么神奇的事情，仅仅是组装成完整的SQL语句，和参数数组一起，实例化SQLiteStatement，然后执行这个语句。
执行的过程实现在SQLiteStatement.java。
/**
* Execute this SQL statement, if the the number of rows affected by execution of this SQL
* statement is of any importance to the caller - for example, UPDATE / DELETE SQL statements.
*
* @return the number of rows affected by this SQL statement execution.
* @throws android.database.SQLException If the SQL string is invalid for
* some reason
*/
public int executeUpdateDelete() {
acquireReference();
try {
// 获取各个参数：sql语句、要绑定的参数等
// 然后才是真正的执行
return getSession().executeForChangedRowCount(
getSql(), getBindArgs(), getConnectionFlags(), null);
} catch (SQLiteDatabaseCorruptException ex) {
onCorruption();
throw ex;
} finally {
releaseReference();
}
}
获取参数，然后调用executeForChangedRowCount方法。这个方法在SQLiteSession.java。
/**
* Executes a statement that returns a count of the number of rows
* that were changed. Use for UPDATE or DELETE SQL statements.
*
* @param sql The SQL statement to execute.
* @param bindArgs The arguments to bind, or null if none.
* @param connectionFlags The connection flags to use if a connection must be
* acquired by this operation. Refer to {@link SQLiteConnectionPool}.
* @param cancellationSignal A signal to cancel the operation in progress, or null if none.
* @return The number of rows that were changed.
*
* @throws SQLiteException if an error occurs, such as a syntax error
* or invalid number of bind arguments.
* @throws OperationCanceledException if the operation was canceled.
*/
public int executeForChangedRowCount(String sql, Object[] bindArgs, int connectionFlags,
CancellationSignal cancellationSignal) {
if (sql == null) {
throw new IllegalArgumentException("sql must not be null.");
}
// 这里虽然传入了bindArgs，但并没用到
if (executeSpecial(sql, bindArgs, connectionFlags, cancellationSignal)) {
return 0;
}
acquireConnection(sql, connectionFlags, cancellationSignal); // might throw
try {
// 真正用到sql和bindArgs的地方
return mConnection.executeForChangedRowCount(sql, bindArgs,
cancellationSignal); // might throw
} finally {
releaseConnection(); // might throw
}
}
Ok，继续深入，来到SQLiteConnection.java。
/**
* Executes a statement that returns a count of the number of rows
* that were changed. Use for UPDATE or DELETE SQL statements.
*
* @param sql The SQL statement to execute.
* @param bindArgs The arguments to bind, or null if none.
* @param cancellationSignal A signal to cancel the operation in progress, or null if none.
* @return The number of rows that were changed.
*
* @throws SQLiteException if an error occurs, such as a syntax error
* or invalid number of bind arguments.
* @throws OperationCanceledException if the operation was canceled.
*/
public int executeForChangedRowCount(String sql, Object[] bindArgs,
CancellationSignal cancellationSignal) {
if (sql == null) {
throw new IllegalArgumentException("sql must not be null.");
}
int changedRows = 0;
final int cookie = mRecentOperations.beginOperation("executeForChangedRowCount",
sql, bindArgs);
try {
// 获取预先编译过的SQL
final PreparedStatement statement = acquirePreparedStatement(sql);
try {
throwIfStatementForbidden(statement);
// 参数绑定
bindArguments(statement, bindArgs);
applyBlockGuardPolicy(statement);
attachCancellationSignal(cancellationSignal);
try {
// 交给SQLiteEngine执行
changedRows = nativeExecuteForChangedRowCount(
mConnectionPtr, statement.mStatementPtr);
return changedRows;
} finally {
detachCancellationSignal(cancellationSignal);
}
} finally {
releasePreparedStatement(statement);
}
} catch (RuntimeException ex) {
mRecentOperations.failOperation(cookie, ex);
throw ex;
} finally {
if (mRecentOperations.endOperationDeferLog(cookie)) {
mRecentOperations.logOperation(cookie, "changedRows=" + changedRows);
}
}
}
首先，会通过acquirePreparedStatement去获取PreparedStatement实例，源码如下：
private PreparedStatement acquirePreparedStatement(String sql) {
PreparedStatement statement = mPreparedStatementCache.get(sql);
boolean skipCache = false;
if (statement != null) {
if (!statement.mInUse) {
return statement;
}
// The statement is already in the cache but is in use (this statement appears
// to be not only re-entrant but recursive!). So prepare a new copy of the
// statement but do not cache it.
skipCache = true;
}
final long statementPtr = nativePrepareStatement(mConnectionPtr, sql);
try {
final int numParameters = nativeGetParameterCount(mConnectionPtr, statementPtr);
final int type = DatabaseUtils.getSqlStatementType(sql);
final boolean readOnly = nativeIsReadOnly(mConnectionPtr, statementPtr);
statement = obtainPreparedStatement(sql, statementPtr, numParameters, type, readOnly);
if (!skipCache && isCacheable(type)) {
mPreparedStatementCache.put(sql, statement);
statement.mInCache = true;
}
} catch (RuntimeException ex) {
// Finalize the statement if an exception occurred and we did not add
// it to the cache. If it is already in the cache, then leave it there.
if (statement == null || !statement.mInCache) {
nativeFinalizeStatement(mConnectionPtr, statementPtr);
}
throw ex;
}
statement.mInUse = true;
return statement;
}
可以看到，这里有个mPreparedStatementCache用于缓存之前生成过的PreparedStatement，如果之前有相同的SQL语句，则取出重用，避免重复编译SQL。这个缓存本质上是一个LruCache<String, PreparedStatement>，key为sql语句。
也即是，如果我们使用whereClause和whereArgs的方式操作数据库的话，同样的whereClause，不同的whereArgs取值，将能利用到这个缓存。但如果直接将限定语句拼接好，通常情况下，参数取值会改变，已有的缓存就派不上用场，白白浪费了已有的PreparedStatement缓存。
顺便贴下绑定参数的代码：
private void bindArguments(PreparedStatement statement, Object[] bindArgs) {
final int count = bindArgs != null ? bindArgs.length : 0;
if (count != statement.mNumParameters) {
throw new SQLiteBindOrColumnIndexOutOfRangeException(
"Expected " + statement.mNumParameters + " bind arguments but "
+ count + " were provided.");
}
if (count == 0) {
return;
}
final long statementPtr = statement.mStatementPtr;
for (int i = 0; i < count; i++) {
final Object arg = bindArgs[i];
switch (DatabaseUtils.getTypeOfObject(arg)) {
case Cursor.FIELD_TYPE_NULL:
nativeBindNull(mConnectionPtr, statementPtr, i + 1);
break;
case Cursor.FIELD_TYPE_INTEGER:
nativeBindLong(mConnectionPtr, statementPtr, i + 1,
((Number)arg).longValue());
break;
case Cursor.FIELD_TYPE_FLOAT:
nativeBindDouble(mConnectionPtr, statementPtr, i + 1,
((Number)arg).doubleValue());
break;
case Cursor.FIELD_TYPE_BLOB:
nativeBindBlob(mConnectionPtr, statementPtr, i + 1, (byte[])arg);
break;
case Cursor.FIELD_TYPE_STRING:
default:
if (arg instanceof Boolean) {
// Provide compatibility with legacy applications which may pass
// Boolean values in bind args.
nativeBindLong(mConnectionPtr, statementPtr, i + 1,
((Boolean)arg).booleanValue() ? 1 : 0);
} else {
nativeBindString(mConnectionPtr, statementPtr, i + 1, arg.toString());
}
break;
}
}
}
代码很简单，就不多解释了。
啰啰嗦嗦贴了这么多源码，其实只是为了证明，whereClause搭配whereArgs是很有意义的。为了性能考虑，写代码的时候不要再用拼接字符串的方式直接生成限定语句了。
但，最开始提及的那种不便的使用方式，难道就只能默默忍受了？答案显然并不是，通过简单的抽象、封装，能够实现如下的效果：
Statement statement =
Statement.where(UPDATE_TIME).lessOrEqual(now)
.and(EXPIRY_TIME).moreThan(now)
.or(AGE).eq(23)
.end();
statement.sql(); // 生成sql语句
statement.whereClause(); // 生成whereClause语句
statement.args(); // 对应的参数数组
这是我尝试造的一个轮子，用于通过语义化的方式，定义和生成whereClause和whereArgs。用起来就像是写sql语句一样自然，同时还能避免人工书写sql语句导致的一些拼写错误，生成的whereClause的参数顺序也和whereArgs参数数组严格对应。
找时间整理下，分享到Gayhub供大家批判下。哈哈。
