DruidCP源码阅读5 -- PSCache

1、开启PSCache

// 开启
dataSource.setPoolPreparedStatements(true);
dataSource.setMaxPoolPreparedStatementPerConnectionSize(20);

configFromProperty加载配置文件的时候，设置每个连接指定的PSCache缓存的大小，并且配置poolPreparedStatements变量为true，所以如果要开启PSCache除了指定setPoolPreparedStatements=true，还需要指定每个连接的缓存大小配置

{
    String property = properties.getProperty("druid.maxPoolPreparedStatementPerConnectionSize");
    if (property != null && property.length() > 0) {
        try {
            int value = Integer.parseInt(property);
            this.setMaxPoolPreparedStatementPerConnectionSize(value);
        } catch (NumberFormatException e) {
            LOG.error("illegal property 'druid.maxPoolPreparedStatementPerConnectionSize'", e);
        }
    }
}

public void setMaxPoolPreparedStatementPerConnectionSize(int maxPoolPreparedStatementPerConnectionSize) {
    if (maxPoolPreparedStatementPerConnectionSize > 0) {
        this.poolPreparedStatements = true;
    } else {
        this.poolPreparedStatements = false;
    }

    this.maxPoolPreparedStatementPerConnectionSize = maxPoolPreparedStatementPerConnectionSize;
}

预编译sql的时候如果开启了PSCache，会对cache进行初始化

Connection conn = getConnection();
PreparedStatement stmt = conn.prepareStatement("select * from depart where id =?");
stmt.setInt(1,1);
ResultSet resultSet = stmt.executeQuery();

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在获取缓存的时候通过key来获取，这个key是PreparedStatementKey一个静态内部类

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通过key获取PreparedStatementHolder对象，其中这个map是一个LRUCache，即清除缓存的策略是不常用的缓存会被清除，也是继承LinkedHashMap实现的缓存策略

PreparedStatementHolder holder = map.get(key);

public PreparedStatementHolder get(PreparedStatementKey key) throws SQLException {
        // 获取到缓存数据
        PreparedStatementHolder holder = map.get(key);

        if (holder != null) {
            if (holder.isInUse() && (!dataSource.isSharePreparedStatements())) {
                return null;
            }

            // 命中缓存记录数 + 1
            holder.incrementHitCount();
            dataSource.incrementCachedPreparedStatementHitCount();
            if (holder.isEnterOracleImplicitCache()) {
                OracleUtils.exitImplicitCacheToActive(holder.statement);
            }
        } else {
            dataSource.incrementCachedPreparedStatementMissCount();
        }

        return holder;
    }

2、map.get(key)

get(key)时会调用当前key的hashcode，PreparedStatementKey对象也是重写了hashCode()，这个方法会把PreparedStatementKey实例的所有成员变量都进行累加，也就是在get(key)的时候要比较当前实例实例的所有变量值

public static class PreparedStatementKey {
        //省略

        。。。

        public int hashCode() {
            final int prime = 31;
            int result = 1;
            result = prime * result + ((sql == null) ? 0 : sql.hashCode());
            result = prime * result + ((catalog == null) ? 0 : catalog.hashCode());
            result = prime * result + ((methodType == null) ? 0 : methodType.hashCode());

            result = prime * result + resultSetConcurrency;
            result = prime * result + resultSetHoldability;
            result = prime * result + resultSetType;

            result = prime * result + autoGeneratedKeys;

            result = prime * result + Arrays.hashCode(columnIndexes);
            result = prime * result + Arrays.hashCode(columnNames);

            System.out.println(result);
            return result;
        }

        public String getSql() {
            return sql;
        }

    }