数据库 connection 的本质是什么

目前在网上看到有如下两种获取 connection 的办法：

第一种：
//3.创建Driver对象
Driver driver = (Driver)Class.forName(driverName).newInstance();

Properties info = new Properties(); //driver的connect方法中需要一个Properties型的参数
info.put("user", user);
info.put("password", password);

//4.使用driver的connect方法获取数据库连接
Connection conn = driver.connect(url, info);


第二种：
/3.加载驱动程序，即将数据库厂商提供的Driver接口实现类加载进内存；
//  该驱动类中的静态代码块包含有注册驱动的程序，在加载类时将被执行
Class.forName(driverName);

//4.通过DriverManager类的静态方法getConnection获取数据连接
Connection conn = DriverManager.getConnection(url, user, password);


总结来说就算一种是通过 Dirver 来获取，一种是通过 DriverManager 来获取.

然后进一步调试源码，发现通过 DriverManager 来获取，最终还是转换成通过 Driver 来获取，如下：



所以，这两者本质上是一个东西.

那下面在说下 connection 是什么？或者说 connection 有哪些东西？

说这个之前，我们先看下 Driver 是啥？或者说我们先自定义一个 Driver.

public class SSDriver implements Driver {
    private final String URL_PREFIX = "jdbc:smalldb";

    static SSDriver ssDriver;

    static {
        ssDriver = new SSDriver();
        try {
            DriverManager.registerDriver(ssDriver);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    // 解析 url
    // 例：jdbc:mysql://192.168.0.105:3306/shgb_fz?useUnicode=true&characterEncoding=UTF8&
    // autoReconnect=true&zeroDateTimeBehavior=convertToNull
    private Properties parse(String url, Properties info) throws SQLException {
        Properties props = (Properties)info.clone();
        if(!acceptsURL(url)){
            return props;
        }
        int idx1 = url.indexOf(':', 5); // search after "jdbc:"
        int idx2 = url.indexOf('?');
        if(idx1 > 0){
            String dbPath = (idx2 > 0) ? url.substring(idx1 + 1, idx2) : url.substring(idx1 + 1);
            props.setProperty("dbpath", dbPath);
        }
        if(idx2 > 0){
            String propsString = url.substring(idx2 + 1);
            StringTokenizer tok = new StringTokenizer(propsString, ";");
            while(tok.hasMoreTokens()){
                String keyValue = tok.nextToken().trim();
                if(keyValue.length() > 0){
                    idx1 = keyValue.indexOf('=');
                    if(idx1 > 0){
                        String key = keyValue.substring(0, idx1).toLowerCase().trim();
                        String value = keyValue.substring(idx1 + 1).trim();
                        props.put(key, value);
                    }else{
                        throw SmallDBException.create(Language.CUSTOM_MESSAGE, "Missing equal in property:" + keyValue);
                    }
                }
            }
        }
        return props;
    }

    public Connection connect(String url, Properties info) throws SQLException {
        if(acceptsURL(url)){
            return new SSConnection(parse(url, info));
        }
        return null;
    }

    // 检查该 URL 是否合法.
    public boolean acceptsURL(String url) throws SQLException {
        return url.startsWith(URL_PREFIX);
    }

    public DriverPropertyInfo[] getPropertyInfo(String url, Properties info) throws SQLException {
        Properties props = parse(url, info);
        DriverPropertyInfo[] driverInfos = new DriverPropertyInfo[1];
        driverInfos[0] = new DriverPropertyInfo("dbpath", props.getProperty("dbpath"));
        return driverInfos;
    }

    public int getMajorVersion() {
        return 0;
    }

    public int getMinorVersion() {
        return 20;
    }

    public boolean jdbcCompliant() {
        return true;
    }

    public Logger getParentLogger() throws SQLFeatureNotSupportedException {
        return null;
    }
}

然后看下 connection 中做了那些事：

/**
* Connection 应该包含哪些信息了？
* 1.连接那个数据库
* 2.事务隔离级别
* 3.是否自动提交
*/
public class SSConnection implements Connection {

    private boolean autoCommit = true;
    private Database database;
    int isolationLevel = TRANSACTION_READ_COMMITTED;
    // 待提交的集合.
    private List commitPages = new ArrayList();
    // 事务开启时间
    private long transactionTime;

    private final SSDatabaseMetadata metadata;

    private int holdability;

    final Logger log;

    public SSConnection(Properties props) throws SQLException {
        SmallDBException.setLanguage(props.get("locale"));
        log = new Logger();
        String name = props.getProperty("dbpath");
        boolean create = "true".equals(props.getProperty("create"));
        database = Database.getDatabase(name, this, create);
        metadata = new SSDatabaseMetaData(this);
    }

    SSConnection(SSConnection conn){
        database = conn.database;
        metadata = conn.metadata;
        log = conn.log;
    }

    Database getDatabase(boolean returnNull) throws SQLException{
        testClosedConnection();
        // 打开数据的含义就是读文件.
        // 数据库数据的存储本质上也是文件的存储.
        if(!returnNull && database == null) throw SmallDBException.create(Language.DB_NOTCONNECTED);
        return database;
    }

    final void testClosedConnection() throws SQLException{
        if(isClosed()) throw SmallDBException.create(Language.CONNECTION_CLOSED);
    }

    Object getMonitor(){
        return this;
    }

    @Override
    public Statement createStatement() throws SQLException {
        return new SSStatement(this);
    }

    @Override
    public PreparedStatement prepareStatement(String sql) throws SQLException {
        return SSPreparedStatement(this, sql);
    }

    @Override
    public CallableStatement prepareCall(String sql) throws SQLException {
        return SSCallableStatement(this, sql);
    }

    @Override
    public String nativeSQL(String sql) throws SQLException {
        return sql;
    }

    @Override
    public void setAutoCommit(boolean autoCommit) throws SQLException {
        if(log.isLogging()) log.println("AutoCommit:"+autoCommit);
        if(this.autoCommit != autoCommit){
            // 先提交，后改变
            commit();
            this.autoCommit = autoCommit;
        }
    }

    @Override
    public boolean getAutoCommit() throws SQLException {
        return autoCommit;
    }

    /**
     * Add a page for later commit or rollback.
     */
    void add(TransactionStep storePage) throws SQLException{
        testClosedConnection();
        synchronized(getMonitor()){
            commitPages.add(storePage);
        }
    }

    // TODO 待定
    @Override
    public void commit() throws SQLException {
        log.println("Commit");
        testClosedConnection();
        synchronized(getMonitor()){
            try{
                int count = commitPages.size();
                for(int i=0; i<count; i++){
                    TransactionStep page = (TransactionStep)commitPages.get(i);
page.commit();
                }
                for(int i=0; i<count; i++){
                    TransactionStep page = (TransactionStep)commitPages.get(i);
                    page.freeLock();
                }
                commitPages.clear();
                transactionTime = System.currentTimeMillis();
            }catch(Throwable e){
                rollback();
                throw SmallDBException.createFromException(e);
            }
        }
    }

    /**
     * Discard all changes of a file because it was deleted.
     */
    void rollbackFile(RandomAccessFile raFile) throws SQLException{
        testClosedConnection();
        // remove the all commits that point to this table
        synchronized(getMonitor()){
            for(int i = commitPages.size() - 1; i >= 0; i--){
                TransactionStep page = (TransactionStep)commitPages.get(i);
                if(page.raFile == raFile){
                    page.rollback();
                    page.freeLock();
                }
            }
        }
    }

    void rollback(int savepoint) throws SQLException{
        testClosedConnection();
        synchronized(getMonitor()){
            for(int i = commitPages.size() - 1; i >= savepoint; i--){
                TransactionStep page = (TransactionStep)commitPages.remove(i);
                page.rollback();
                page.freeLock();
            }
        }
    }

    @Override
    public void rollback() throws SQLException {
        log.println("Rollback");
        testClosedConnection();
        synchronized(getMonitor()){
            int count = commitPages.size();
            for(int i=0; i<count; i++){
                TransactionStep page = (TransactionStep)commitPages.get(i);
                page.rollback();
                page.freeLock();
            }
            commitPages.clear();
            transactionTime = System.currentTimeMillis();
        }
    }

    // 关闭前回滚
    @Override
    public void close() throws SQLException {
        rollback();
        database = null;
        commitPages = null;
        Database.closeConnection(this);
    }

    @Override
    public boolean isClosed() throws SQLException {
        return null == commitPages;
    }

    @Override
    public DatabaseMetaData getMetaData() throws SQLException {
        return metadata;
    }

    @Override
    public void setReadOnly(boolean readOnly) throws SQLException {
        //TODO Connection ReadOnly implementing
    }

    @Override
    public boolean isReadOnly() throws SQLException {
        return false;
    }

    @Override
    public void setCatalog(String catalog) throws SQLException {
        testClosedConnection();
        database = Database.getDatabase(catalog, this, false);
    }

    @Override
    public String getCatalog() throws SQLException {
        if(database == null)
            return "";
        return database.getName();
    }

    @Override
    public void setTransactionIsolation(int level) throws SQLException {
        if(!metadata.supportsTransactionIsolationLevel(level)) {
            throw SmallSQLException.create(Language.ISOLATION_UNKNOWN, String.valueOf(level));
        }
        isolationLevel = level;
    }

    @Override
    public int getTransactionIsolation() throws SQLException {
        return isolationLevel;
    }

    @Override
    public SQLWarning getWarnings() throws SQLException {
        return null;
    }

    @Override
    public void clearWarnings() throws SQLException {

    }

    @Override
    public Statement createStatement(int resultSetType, int resultSetConcurrency) throws SQLException {
        return new SSStatement( this, resultSetType, resultSetConcurrency);
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
        return new SSPreparedStatement(this, sql, resultSetType, resultSetConcurrency);
    }

    @Override
    public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency) throws SQLException {
        return new SSCallableStatement(this, sql, resultSetType, resultSetConcurrency);
    }

    @Override
    public Map<String, Class<?>> getTypeMap() throws SQLException {
        return null;
    }

    @Override
    public void setTypeMap(Map<String, Class<?>> map) throws SQLException {

    }

    @Override
    public void setHoldability(int holdability) throws SQLException {
        this.holdability = holdability;
    }

    @Override
    public int getHoldability() throws SQLException {
        return holdability;
    }

    int getSavepoint() throws SQLException{
        testClosedConnection();
        return commitPages.size(); // the call is atomic, that it need not be synchronized
    }

    @Override
    public Savepoint setSavepoint() throws SQLException {
        return new SSSavepoint(getSavepoint(), null, transactionTime);;
    }

    @Override
    public Savepoint setSavepoint(String name) throws SQLException {
        return new SSSavepoint(getSavepoint(), name, transactionTime);
    }

    @Override
    public void rollback(Savepoint savepoint) throws SQLException {

    }

    @Override
    public void releaseSavepoint(Savepoint savepoint) throws SQLException {

    }

    @Override
    public Statement createStatement(int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
        return null;
    }

    @Override
    public CallableStatement prepareCall(String sql, int resultSetType, int resultSetConcurrency, int resultSetHoldability) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int autoGeneratedKeys) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, int[] columnIndexes) throws SQLException {
        return null;
    }

    @Override
    public PreparedStatement prepareStatement(String sql, String[] columnNames) throws SQLException {
        return null;
    }

    @Override
    public Clob createClob() throws SQLException {
        return null;
    }

    @Override
    public Blob createBlob() throws SQLException {
        return null;
    }

    @Override
    public NClob createNClob() throws SQLException {
        return null;
    }

    @Override
    public SQLXML createSQLXML() throws SQLException {
        return null;
    }

    @Override
    public boolean isValid(int timeout) throws SQLException {
        return false;
    }

    @Override
    public void setClientInfo(String name, String value) throws SQLClientInfoException {

    }

    @Override
    public void setClientInfo(Properties properties) throws SQLClientInfoException {

    }

    @Override
    public String getClientInfo(String name) throws SQLException {
        return null;
    }

    @Override
    public Properties getClientInfo() throws SQLException {
        return null;
    }

    public Array createArrayOf(String typeName, Object[] elements) throws SQLException {
        return null;
    }

    public Struct createStruct(String typeName, Object[] attributes) throws SQLException {
        return null;
    }

    public void setSchema(String schema) throws SQLException {

    }

    public String getSchema() throws SQLException {
        return null;
    }

    public void abort(Executor executor) throws SQLException {

    }

    public void setNetworkTimeout(Executor executor, int milliseconds) throws SQLException {

    }

    public int getNetworkTimeout() throws SQLException {
        return 0;
    }

    @Override
    public <T> T unwrap(Class<T> iface) throws SQLException {
        return null;
    }

    @Override
    public boolean isWrapperFor(Class<?> iface) throws SQLException {
        return false;
    }
}