How to read version number from a database file in Android which is placed in asset folder(如何从放在资产文件夹中的Android数据库文件中读取版本号)

本文介绍了如何从放在资产文件夹中的Android数据库文件中读取版本号的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

在我手动放置的包数据库下的ASSET文件夹中有一个SQLite文件(如下所示：SRC->Main->ASSET->数据库->Q-db(Q-db是我的SQLite文件)。现在我想读取它的版本号，如果这个版本放在我自己的应用程序中，我想要匹配，如果不是，那么这个需要复制到我的数据库中，否则将跳过。当然，第一次它会从文件中复制，下一次打开应用程序时，它会检查版本号。

我谷歌了很多次，但无法获得文件的版本号，我得到的是我的SQLite版本号，在我的情况下，我不想要它。我们将非常感谢您的支持。

参考：如下所示：

https://www.concretepage.com/android/android-ship-sqlite-database-with-apk-copy-sqlite-database-from-assets-folder-to-data-example

User_Version

如前所述，您可能正在谈论User_Version。首先要注意的是，它是一个用户控制的变量/字段，可供自定义使用。SQlite不使用或更改USER_VERSION，但允许更改和使用它。

此外，SQLite管理器(如DB浏览器、Navicat等)不会自动更改版本号。因此，在将数据库文件复制到Assets文件夹之前，您必须故意更改USER_VERSION以使其可用(请注意，如果这样做并且您使用的是SQLiteOpenHelper的子类，则可能会调用onUpgrade和onDowngrade方法)。

如果没有特别更改USER_VERSION，并且数据库仅由SQLite管理器工具访问，则其USER_VERSION将为0。如果数据库文件是通过从使用SQLiteOpenHelper的子类的Android应用程序复制数据库文件来打开的，则它的USER_VERSION将为1或更大(取决于用作SQLiteOpenHelper的构造器的第四个参数的最后一个值)。当然，如果以编程方式更改了USER_VERSION，则如果将文件复制到SQlite管理器工具中，也会反映这样的更改。

在复制文件之前，通常会在SQlite管理器工具中将USER_VERSION更改为适当的值。

您可以使用SQLPRAGMA user_version = 5;更改User_Version 您可以使用PRAGMA user_version或SELECT * FROM pragma_user_version;检索USER_VERSION

如果您需要在打开数据库之前检查版本，那么您可以读取偏移量为60的4个字节，并将这4个字节转换为一个整数，以对照另一个值检查USER_VERSION。否则，您可能必须从Assets文件夹复制文件(可能使用不同的名称)，将其作为SQLiteDatabase打开，并使用上面的SQL检索USER_VERSION，然后对照其他值进行检查，关闭数据库文件。如果不需要，则删除该文件，否则删除先前的数据库文件，然后重命名复制的文件。

示例

以下是一个工作示例(请注意，我很少使用Kotlin，这是使用as Studio从Java转换而来的)。

这使用了一个类，即SQLAssetVersionCheck，它从文件中提取版本号，而不是将文件作为SQLiteDatabase打开。

SQLAssetVersionCheck.kt：-

class SQLAssetVersionCheck
/**
 * Full SQLAssetVersionCheck Constructor - sub directories can be specified
 * @param context           Assets are part of package so use the context to get the asset file
 * @param dbName            The database name (i.e. the file name)
 * @param subDirectories    The sub-directories as per the heirarchial order
 * @param dbVersion         The database version to check against
 */
(context: Context, val databaseName: String, subDirectories: Array<String>?, dbVersion: Int) {
    val assetPath: String
    var databaseVersion: Int = 0
        private set
    var result: Int = 0
        private set


    init {
        assetPath = applySubDirectories(databaseName, subDirectories)
        Log.d("SQLAVC", "Looking for Asset $assetPath")
        var stage = 0
        try {
            val `is` = context.assets.open(assetPath)
            stage++
            // Get the first 64 bytes of the header
            val v = ByteArray(64)
            `is`.read(v, 0, 64)
            // only interested in the 4 bytes from offset 60 so get them
            val v2 = ByteArray(4)
            for (i in 60..63) {
                v2[i - 60] = v[i]
            }
            stage++
            // Done with the InputStream so close it
            `is`.close()
            // Extarct the stored DBVersion
            databaseVersion = ByteBuffer.wrap(v2).int
            if (databaseVersion < dbVersion) {
                result = ASSETVERSIONLOW

            }
            if (databaseVersion > dbVersion) {
                result = ASSETVERSIONHIGH
            }
            if (databaseVersion == dbVersion) {
                result = ASSETVERSIONMATCH
            }

        } catch (e: IOException) {
            e.printStackTrace()
            when (stage) {
                0 -> result = ASSETNOTFOUND
                1 -> result = ASSETIOERROR
            }
        }

    }

    constructor(context: Context, dbName: String, dbVersion: Int) : this(context, dbName, null, dbVersion) {}

    private fun applySubDirectories(dbname: String, subDirectories: Array<String>?): String {
        val base = StringBuffer("")
        var firstdirectory = true
        if (subDirectories != null) {
            for (d in subDirectories) {
                if (!firstdirectory) {
                    base.append(File.separatorChar)
                }
                firstdirectory = false
                base.append(d)
            }
        }
        if (base.length > 0) {
            base.append(File.separatorChar)
        }
        base.append(dbname)
        return base.toString()
    }

    companion object {

        val ASSETNOTFOUND = -2
        val ASSETIOERROR = -3
        val ASSETVERSIONMATCH = 0
        val ASSETVERSIONHIGH = 1
        val ASSETVERSIONLOW = -1
    }
}

下面是一个活动，它两次使用上面的类来尝试检查testdb文件中的版本。

第一次使用时找不到数据库文件testdb，因为它在Assets文件夹(不是数据库子目录)中查找。
第二次用法是在指定子目录数据库(完整构造函数的第三个参数)时找到testdb文件，在sets/Databages/文件夹中找到：-

MainActivity.kt：-

class MainActivity : AppCompatActivity() {


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val db_version_to_check_against = 100

        var mAVC1 = SQLAssetVersionCheck(this, "testdb", 100)

        var result = ""
        when (mAVC1.result) {
            SQLAssetVersionCheck.ASSETIOERROR -> result = "IO ERROR detected - check the Log"
            SQLAssetVersionCheck.ASSETNOTFOUND -> result = "The Asset, for Database " + mAVC1.databaseName + " was not located at  " + mAVC1.assetPath
            SQLAssetVersionCheck.ASSETVERSIONHIGH -> result = "The Asset was located and the version number being " +
                    mAVC1.databaseVersion.toString() +
                    " was higher than the version to be checked which was " +
                    db_version_to_check_against.toString()
            SQLAssetVersionCheck.ASSETVERSIONLOW -> result = "The Asset was located and the version number being " +
                    mAVC1.databaseVersion.toString() +
                    " was lower than the version to be checked which was " +
                    db_version_to_check_against.toString()
            SQLAssetVersionCheck.ASSETVERSIONMATCH -> result = "The Asset version and the version to be check ed are the same."
        }
        Log.d("ASSETVERSIONCHECK", "The result of the version check was - $result")

        var mAVC2 = SQLAssetVersionCheck(this, "testdb", arrayOf("databases"), db_version_to_check_against)
        result = ""
        when (mAVC2.result) {
            SQLAssetVersionCheck.ASSETIOERROR -> result = "IO ERROR detected - check the Log"
            SQLAssetVersionCheck.ASSETNOTFOUND -> result = "The Asset, for Database " + mAVC2.databaseName + " was not located at  " + mAVC2.assetPath
            SQLAssetVersionCheck.ASSETVERSIONHIGH -> result = "The Asset was located and the version number being " +
                    mAVC2.databaseVersion.toString() +
                    " was higher than the version to be checked which was " +
                    db_version_to_check_against.toString()
            SQLAssetVersionCheck.ASSETVERSIONLOW -> result = "The Asset was located and the version number being " +
                    mAVC2.databaseVersion.toString() +
                    " was lower than the version to be checked which was " +
                    db_version_to_check_against.toString()
            SQLAssetVersionCheck.ASSETVERSIONMATCH -> result = "The Asset version and the version to be check ed are the same."
        }
        Log.d("ASSETVERSIONCHECK", "The result of the version check was - $result")
    }
}

结果(日志)：-

2019-02-19 13:11:34.473 19058-19058/com.example.so54741423assetdbversioning D/SQLAVC: Looking for Asset testdb
2019-02-19 13:11:34.473 19058-19058/com.example.so54741423assetdbversioning W/System.err: java.io.FileNotFoundException: testdb
2019-02-19 13:11:34.474 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.content.res.AssetManager.nativeOpenAsset(Native Method)
2019-02-19 13:11:34.474 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.content.res.AssetManager.open(AssetManager.java:744)
2019-02-19 13:11:34.474 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.content.res.AssetManager.open(AssetManager.java:721)
2019-02-19 13:11:34.474 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at com.example.so54741423assetdbversioning.SQLAssetVersionCheck.<init>(SQLAssetVersionCheck.kt:31)
2019-02-19 13:11:34.474 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at com.example.so54741423assetdbversioning.SQLAssetVersionCheck.<init>(SQLAssetVersionCheck.kt:67)
2019-02-19 13:11:34.474 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at com.example.so54741423assetdbversioning.MainActivity.onCreate(MainActivity.kt:17)
2019-02-19 13:11:34.474 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.Activity.performCreate(Activity.java:7136)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.Activity.performCreate(Activity.java:7127)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:106)
2019-02-19 13:11:34.475 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.os.Looper.loop(Looper.java:193)
2019-02-19 13:11:34.476 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:6669)
2019-02-19 13:11:34.476 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
2019-02-19 13:11:34.476 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
2019-02-19 13:11:34.476 19058-19058/com.example.so54741423assetdbversioning W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
2019-02-19 13:11:34.476 19058-19058/com.example.so54741423assetdbversioning D/ASSETVERSIONCHECK: The result of the version check was - The Asset, for Database testdb was not located at  testdb




2019-02-19 13:11:34.476 19058-19058/com.example.so54741423assetdbversioning D/SQLAVC: Looking for Asset databases/testdb
2019-02-19 13:11:34.477 19058-19058/com.example.so54741423assetdbversioning D/ASSETVERSIONCHECK: The result of the version check was - The Asset was located and the version number being 5 was lower than the version to be checked which was 100

第一次尝试找不到该文件(显示捕获的异常)，并显示要显示的行版本检查的结果是-数据库Testdb的资产未位于Testdb
第二次尝试有效并导致版本检查的结果是-资产已找到，版本号为5的版本低于要检查的版本100
添加空行间隙是为了将第二次尝试与第一次尝试分开。

其他

使用SQLite管理器工具(Navicat)并使用：-

PRAGMA user_version = 101;

然后将文件(在Navicat中关闭连接后)复制到Assets文件夹(因此我有两个testdb文件)，结果是：-

2019-02-19 13:50:09.874 19253-19253/com.example.so54741423assetdbversioning D/SQLAVC: Looking for Asset testdb
2019-02-19 13:50:09.874 19253-19253/com.example.so54741423assetdbversioning D/ASSETVERSIONCHECK: The result of the version check was - The Asset was located and the version number being 101 was higher than the version to be checked which was 100
2019-02-19 13:50:09.874 19253-19253/com.example.so54741423assetdbversioning D/SQLAVC: Looking for Asset databases/testdb
2019-02-19 13:50:09.874 19253-19253/com.example.so54741423assetdbversioning D/ASSETVERSIONCHECK: The result of the version check was - The Asset was located and the version number being 5 was lower than the version to be checked which was 100