QSqlDatabase在多线程中的使用

Qt中多线程使用数据库_qt数据库管理类支持多数据库,多线程-CSDN博客

1.

代码：

#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 += query.value(0).toString() + "|";m_database.close();emit query_result(str + "\n");}
private:QSqlDatabase m_database;
};class Widget : public QWidget
{Q_OBJECTpublic:Widget(QWidget *parent = nullptr):   QWidget(parent){QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", "mydb");db.setPort(3306);db.setDatabaseName("test");db.setUserName("root");db.setPassword("root_pwd");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

数据库： 

1000条记录：内容均为123

ui： 

 

有一个线程中的数据库没有打开

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2.

修改代码：

#ifndef WIDGET_H
#define WIDGET_H#include <QWidget>
#include <QPushButton>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QSqlDriver>
#include <QThread>
#include <QTextBrowser>
#include <QVBoxLayout>
#include <QDebug>
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(){qDebug()<<__FILE__<<"["<<__LINE__<<"]"<<m_database;m_database.open();qDebug()<<__FILE__<<"["<<__LINE__<<"]"<<m_database;QString str = "Thread ID: " + QString::number((int)QThread::currentThreadId(), 16) + "  result:";QSqlQuery query(m_database);query.exec("SELECT * FROM test;");while(query.next())str += query.value(0).toString() + "|";m_database.close();emit query_result(str + "\n");}
private:QSqlDatabase m_database;
};class Widget : public QWidget
{Q_OBJECTpublic:Widget(QWidget *parent = nullptr):   QWidget(parent){QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", "mydb000");db.setPort(3306);db.setDatabaseName("test");db.setUserName("root");db.setPassword("root_pwd");QSqlDatabase db1 = QSqlDatabase::addDatabase("QMYSQL", "mydb111");db1.setPort(3306);db1.setDatabaseName("test");db1.setUserName("root");db1.setPassword("root_pwd");test1 = new Test(db);test2 = new Test(db1);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

不同的线程中使用不同的连接名：

发现还是不可以成功访问。

debug\../../demo/widget.h [ 27 ] QSqlDatabase(driver="QMYSQL", database="test", host="", port=3306, user="root", open=false)

debug\../../demo/widget.h [ 27 ] QSqlDatabase(driver="QMYSQL", database="test", host="", port=3306, user="root", open=false)

debug\../../demo/widget.h [ 29 ] QSqlDatabase(driver="QMYSQL", database="test", host="", port=3306, user="root", open=false)

QSqlQuery::exec: database not open

debug\../../demo/widget.h [ 29 ] QSqlDatabase(driver="QMYSQL", database="test", host="", port=3306, user="root", open=true)

只有一个数据库可以打开。。。

3.

Qt多线程操作MySql数据库 - 补码 - 博客园 (cnblogs.com)

看了看这篇文章，在

QSqlDatabase::addDatabase和QSqlDatabase::open外部加锁，然后两个数据库都可以打开了。

（应该是在执行这两个函数时，会有数据竞争的情况发生）

#ifndef WIDGET_H
#define WIDGET_H#include <QWidget>
#include <QPushButton>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlError>
#include <QSqlDriver>
#include <QThread>
#include <QTextBrowser>
#include <QVBoxLayout>
#include <QDebug>
#include <QMutex>extern QMutex mutex;
class Test : public QObject
{Q_OBJECT
public:Test(const QString &connectName):   QObject(nullptr){mutex.lock();m_database = QSqlDatabase::addDatabase("QMYSQL",connectName);m_database.setHostName("127.0.0.1");m_database.setPort(3306);m_database.setDatabaseName("test");m_database.setUserName("root");m_database.setPassword("root_pwd");mutex.unlock();}
signals:void query_result(const QString &result);
public slots:void test_query(){qDebug()<<__FILE__<<"["<<__LINE__<<"]"<<m_database;mutex.lock();if(m_database.open()){qDebug()<<__FILE__<<"["<<__LINE__<<"]"<<m_database;QString str = "Thread ID: " + QString::number((int)QThread::currentThreadId(), 16) + "  result:";QSqlQuery query(m_database);query.exec("SELECT * FROM test;");while(query.next())str += query.value(0).toString() + "|";m_database.close();emit query_result(str + "\n");}else{qDebug()<<__FILE__<<"["<<__LINE__<<"]"<<m_database.lastError();}mutex.unlock();}
private:QSqlDatabase m_database;
};class Widget : public QWidget
{Q_OBJECTpublic:Widget(QWidget *parent = nullptr):   QWidget(parent){test1 = new Test("db");test2 = new Test("db1");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

#include <QApplication>
#include "widget.h"QMutex mutex;
int main(int argc, char *argv[])
{QApplication a(argc, argv);Widget w;w.show();return a.exec();
}

debug\../../demo/widget.h [ 38 ] QSqlDatabase(driver="QMYSQL", database="test", host="127.0.0.1", port=3306, user="root", open=false)

debug\../../demo/widget.h [ 38 ] QSqlDatabase(driver="QMYSQL", database="test", host="127.0.0.1", port=3306, user="root", open=false)

debug\../../demo/widget.h [ 42 ] QSqlDatabase(driver="QMYSQL", database="test", host="127.0.0.1", port=3306, user="root", open=true)

debug\../../demo/widget.h [ 42 ] QSqlDatabase(driver="QMYSQL", database="test", host="127.0.0.1", port=3306, user="root", open=true)

 ---

在使用多线程时，特别需要注意的就是刚才出现的情况。

如：有两个线程，它们同时要访问一个共有的资源，这时候就会出现对资源的竞争，如果不添加锁，会出现什么情况将是未知的。

比如上面的例子：

m_database.open()

线程1打开数据库成功，它的流程正常进行。

而线程2打开数据库失败，此时让流程继续走下去，结果不是我们期望的，所以要加锁。