Qt中多线程使用数据库

最近在多线程环境中使用数据库，结果出现了一些比较有意思的问题，然后也找到了原因和解决的办法，这里记录、分享一下过程。

开始我打算开两个线程，然后每个线程进行自己的数据库查询，

然后出现了一个线程能查询到数据，一个不能，甚至程序直接崩溃退出。

如图(后面直接崩溃)：

widget代码如下：

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QPushButton>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QSqlDriver>
#include <QThread>
#include <QTextBrowser>
#include <QVBoxLayout>

class Test : public QObject
{
    Q_OBJECT

public:
    Test(QSqlDatabase database)
        :   QObject(nullptr), m_database(database)
    {

    }

signals:
    void query_result(const QString &result);

public slots:
    void test_query()
    {
        m_database.open();

        QString str = "Thread ID: " + QString::number((int)QThread::currentThreadId(), 16) + "  result: |";
        QSqlQuery query(m_database);
        query.exec("SELECT * FROM test;");

        while(query.next())
            str += QString::number(query.value(1).toInt()) + ":" + query.value(0).toString() + "|";

        m_database.close();

        emit query_result(str + "\n");
    }

private:
    QSqlDatabase m_database;
};

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr)
        :   QWidget(parent)
    {
        QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
        db.setDatabaseName("test.db");
        db.setHostName("localhost");
        db.setUserName("mps");
        db.setPassword("123456");

        test1 = new Test(db);
        test2 = new Test(db);

        test1->moveToThread(&thread1);
        test2->moveToThread(&thread2);

        thread1.start();
        thread2.start();

        QPushButton *button = new QPushButton("开始", this);
        QTextBrowser *browser = new QTextBrowser(this);
        browser->insertPlainText("UI Thread ID: " + QString::number((int)QThread::currentThreadId(), 16));
        QVBoxLayout *layout = new QVBoxLayout(this);
        layout->addWidget(button);
        layout->addWidget(browser);
        setLayout(layout);

        connect(button, &QPushButton::clicked, this, &Widget::start);
        connect(button, &QPushButton::clicked, this, [=]()
        {
           browser->insertPlainText("\n");
        });
        connect(&thread1, &QThread::finished, &thread1, &QThread::deleteLater);
        connect(&thread2, &QThread::finished, &thread2, &QThread::deleteLater);
        connect(test1, &Test::query_result, this, [=](const QString &result)
        {
           browser->insertPlainText(result);
        });
        connect(test2, &Test::query_result, this, [=](const QString &result)
        {
           browser->insertPlainText(result);
        });

        resize(500, 400);
    }

    ~Widget()
    {
        thread1.wait();
        thread2.wait();
    }

public slots:
    void start()
    {
        QMetaObject::invokeMethod(test1, "test_query");
        QMetaObject::invokeMethod(test2, "test_query");
    }

private:
    QThread thread1;
    QThread thread2;

    Test *test1;
    Test *test2;
};

#endif // WIDGET_H

很明显的原因是出现了竞争，然而最大的问题出在两个线程使用了同一个连接，QSqlDatabase::addDatabase的第二个参数为 const QString &connectionName = QLatin1String(defaultConnection) ，默认值即为默认连接，问题部分如下：

QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
db.setDatabaseName("test.db");
db.setHostName("localhost");
db.setUserName("mps");
db.setPassword("123456");

test1 = new Test(db);
test2 = new Test(db);

因此只需要创建各自不同的连接，更改后的代码如下：

#ifndef WIDGET_H
#define WIDGET_H
#include <QWidget>
#include <QPushButton>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QSqlDriver>
#include <QThread>
#include <QTextBrowser>
#include <QVBoxLayout>

class Test : public QObject
{
    Q_OBJECT

public:
    Test(const QString &connectName)
        :   QObject(nullptr)
    {
        m_database = QSqlDatabase::addDatabase("QSQLITE", connectName);
        m_database.setDatabaseName("test.db");
        m_database.setHostName("localhost");
        m_database.setUserName("mps");
        m_database.setPassword("123456");
    }

signals:
    void query_result(const QString &result);

public slots:
    void test_query()
    {
        m_database.open();

        QString str = "Thread ID: " + QString::number((int)QThread::currentThreadId(), 16) + "  result: |";
        QSqlQuery query(m_database);
        query.exec("SELECT * FROM test;");

        while(query.next())
            str += QString::number(query.value(1).toInt()) + ":" + query.value(0).toString() + "|";

        m_database.close();

        emit query_result(str + "\n");
    }

private:
    QSqlDatabase m_database;
};

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr)
        :   QWidget(parent)
    {
        test1 = new Test("test1");
        test2 = new Test("test2");

        test1->moveToThread(&thread1);
        test2->moveToThread(&thread2);

        thread1.start();
        thread2.start();

        QPushButton *button = new QPushButton("开始", this);
        QTextBrowser *browser = new QTextBrowser(this);
        browser->insertPlainText("UI Thread ID: " + QString::number((int)QThread::currentThreadId(), 16));
        QVBoxLayout *layout = new QVBoxLayout(this);
        layout->addWidget(button);
        layout->addWidget(browser);
        setLayout(layout);

        connect(button, &QPushButton::clicked, this, &Widget::start);
        connect(button, &QPushButton::clicked, this, [=]()
        {
           browser->insertPlainText("\n");
        });
        connect(&thread1, &QThread::finished, &thread1, &QThread::deleteLater);
        connect(&thread2, &QThread::finished, &thread2, &QThread::deleteLater);
        connect(test1, &Test::query_result, this, [=](const QString &result)
        {
           browser->insertPlainText(result);
        });
        connect(test2, &Test::query_result, this, [=](const QString &result)
        {
           browser->insertPlainText(result);
        });

        resize(500, 400);
    }

    ~Widget()
    {
        thread1.wait();
        thread2.wait();
    }

public slots:
    void start()
    {
        QMetaObject::invokeMethod(test1, "test_query");
        QMetaObject::invokeMethod(test2, "test_query");
    }

private:
    QThread thread1;
    QThread thread2;

    Test *test1;
    Test *test2;
};
#endif // WIDGET_H

测试图如下：

从结果来看起来，基本没有问题了，也没有再出现崩溃。

最后，还有一些东西要注意，比如每次sql操作前后都使用open / close，否则如果不close，线程就无法退出，然后文档中说明了addDatabase是线程安全的，但open并没有指出，因此建议在open的部分设置Mutex进行保护，以保证其原子性，这样就不会出现任何问题。