6

我有一个工作代码，可以动态加载具有不同类名的不同类实现。类文件被加载到内存数据库( Apache Derby Db ) 中，并且类加载器从BLOB列中检索.class文件。

我想要做的是，将.class文件作为带有版本列和标志的二进制 BLOB插入IS_ENABLED，然后类加载器将在运行时加载不同版本的类。将有相同数量的已编译类版本的 db 条目，并且只有一个IS_ENABLED标志设置为TRUE的类。

因为我尝试使用自定义类加载器加载相同的类名，所以我得到以下异常；

Exception in thread "main" java.lang.LinkageError: loader (instance of  com/levent/classloader/DerbyServerClassLoader): attempted  duplicate class definition for name: "com/levent/greeter/Greeter"
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at com.levent.classloader.DerbyServerClassLoader.findClass(DerbyServerClassLoader.java:38)
    at com.levent.example.ClientClassLoaderDBVersionDemo.main(ClientClassLoaderDBVersionDemo.java:43)

同一个接口（ Greeter.java ）有两个不同的.class文件（Greeter.class.v1，Greeter.class.v2）（在代码开头插入）

在测试代​​码的开头，从 lib/classes/ 文件夹中检索类文件，并将其作为 blob 二进制数据插入到内存数据库中，然后，.class 文件被数据库顺序检索并加载。加载具有相同名称的类时，发生异常。

我怎么解决这个问题？有没有办法卸载一个类，或者无论如何重新加载一个同名的类？

项目树

在此处输入图像描述

DerbyServerClassLoader.java - 类加载器

package com.levent.classloader;

import java.sql.Blob;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class DerbyServerClassLoader extends ClassLoader {

    private ClassLoader parent;
    private String connectionString;

    public DerbyServerClassLoader(String connectionString) {
        this(ClassLoader.getSystemClassLoader(), connectionString);
    }

    public DerbyServerClassLoader(ClassLoader parent, String connectionString) {
        super(parent);
        this.parent = parent;
        this.connectionString = connectionString;
    }

    @Override
    public Class<?> findClass(String name) throws ClassNotFoundException {
        Class cls = null;

        try {
            cls = parent.loadClass(name);           // Delegate to the parent Class Loader
        } catch (ClassNotFoundException clnfE) {    // If parent fails, try to locate and load the class
            byte[] bytes = new byte[0];
            try {
                bytes = loadClassFromDatabase(name);
            } catch (SQLException sqlE) {
                throw new ClassNotFoundException("Unable to load class", sqlE);
            }
            return defineClass(name, bytes, 0, bytes.length);
        }

        return cls;
    }

    private byte[] loadClassFromDatabase(String name) throws SQLException {
        PreparedStatement pstmt = null;
        Connection connection = null;

        try {
            connection = DriverManager.getConnection(connectionString);

            String sql = "SELECT CLASS FROM CLASSES WHERE CLASS_NAME = ? AND IS_ENABLED = ?";
            pstmt = connection.prepareStatement(sql);
            pstmt.setString(1, name);
            pstmt.setBoolean(2, true);
            ResultSet rs = pstmt.executeQuery();

            if (rs.next()) {
                Blob blob = rs.getBlob(1);
                byte[] data = blob.getBytes(1, (int) blob.length());
                return data;
            }
        } catch (SQLException e) {
            System.out.println("Unexpected exception: " + e.toString());
        } catch (Exception e) {
            System.out.println("Unexpected exception: " + e.toString());
        } finally {
            if (pstmt != null) {
                pstmt.close();
            }

            if(connection != null) {
                connection.close();
            }
        }

        return null;
    }

}

Greet.java - 问候接口

package com.levent.greeter;

public interface Greet {

    public String getGreetMessage();

}

DbSingleton.java - 连接 DerbyDb 的实用程序类

package com.levent.derbyutility;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class DbSingleton {

    private static DbSingleton instance = null;

    private Connection conn = null;

    private DbSingleton() {
        try{
            DriverManager.registerDriver(new org.apache.derby.jdbc.EmbeddedDriver());
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public static DbSingleton getInstance() {
        if(instance == null) {
            synchronized(DbSingleton.class) {
                if(instance == null) {
                    instance = new DbSingleton();
                }
            }
        }

        return instance;
    }

    public Connection getConnection() throws SQLException {
        if(conn == null || conn.isClosed()) {
            synchronized (DbSingleton.class) {
                if(conn == null || conn.isClosed()) {
                    try{
                        //String dbUrl = "jdbc:derby://localhost:1527/myDB;create=true;user=me;password=mine";
                        String dbUrl = "jdbc:derby://localhost:1527/memory:myDB;create=true";

                        conn = DriverManager.getConnection(dbUrl);
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }
        }

        return conn;
    }

}

ClientClassLoaderDBVersionDemo.java - 测试代码

package com.levent.example;

import java.io.ByteArrayInputStream;
import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.Statement;

import com.levent.classloader.DerbyServerClassLoader;
import com.levent.derbyutility.DbSingleton;
import com.levent.greeter.Greet;

public class ClientClassLoaderDBVersionDemo {

    // apache derby in-memory db
    private static final String connectionString = "jdbc:derby://localhost:1527/memory:myDB;create=true";
    private static final String classFileName1 = "Greeter.class.v1";
    private static final String classFileName2 = "Greeter.class.v2";
    private static final String className = "com.levent.greeter.Greeter";

    public static void main(String[] args) {
        prepareClass();

        try {
            Greet greet = null;

            DerbyServerClassLoader cl = new DerbyServerClassLoader(connectionString);

            updateVersion(className, "v1");
            Class clazz1 = cl.findClass(className);
            greet = (Greet) clazz1.newInstance();
            System.out.println("Version 1 Greet.getGreetMessage() : " + greet.getGreetMessage());

            updateVersion(className, "v2");
            Class clazz2 = cl.findClass(className);
            greet = (Greet) clazz2.newInstance();
            System.out.println("Version 2 Greet.getGreetMessage() : " + greet.getGreetMessage());           
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }

    private static void prepareClass() {
        DbSingleton instance = DbSingleton.getInstance();

        Connection conn = null;

        try {
            conn = instance.getConnection();
        } catch (SQLException e1) {
            e1.printStackTrace();
        }

        Statement sta;

        if (conn != null) {
            try {
                sta = conn.createStatement();
                int count = sta
                        .executeUpdate("CREATE TABLE CLASSES (CLASS_NAME VARCHAR(50), CLASS BLOB, IS_ENABLED BOOLEAN, VERSION VARCHAR(10) )");
                System.out.println("CLASSES Table created");
                sta.close();

                sta = conn.createStatement();

                PreparedStatement psta = conn.prepareStatement("INSERT INTO CLASSES (CLASS_NAME, CLASS, IS_ENABLED, VERSION) values (?, ?, ?, ?)");
                byte[] bytes = null;
                InputStream blobObject = null;

                psta.setString(1, className);
                bytes = readJarFileAsByteArray(classFileName1);
                blobObject = new ByteArrayInputStream(bytes); 
                psta.setBlob(2, blobObject, bytes.length);
                psta.setBoolean(3, false);
                psta.setString(4, "v1");
                count = psta.executeUpdate();

                psta.setString(1, className);
                bytes = readJarFileAsByteArray(classFileName2);
                blobObject = new ByteArrayInputStream(bytes); 
                psta.setBlob(2, blobObject, bytes.length);
                psta.setBoolean(3, false);
                psta.setString(4, "v2");
                count += psta.executeUpdate();

                System.out.println(count + " record(s) created.");
                sta.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    private static byte[] readJarFileAsByteArray(String classFileName) {
        Path currentRelativePath = Paths.get("");
        String s = currentRelativePath.toAbsolutePath().toString();

        File file = new File(s + "/lib/classes/" + classFileName);
        byte[] fileData = new byte[(int) file.length()];
        DataInputStream dis;
        try {
            dis = new DataInputStream(new FileInputStream(file));
            dis.readFully(fileData);
            dis.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return fileData;
    }

    private static void updateVersion(String className, String version) {
        DbSingleton instance = DbSingleton.getInstance();

        Connection conn = null;

        try {
            conn = instance.getConnection();
        } catch (SQLException e1) {
            e1.printStackTrace();
        }

        Statement sta;

        if (conn != null) {
            try {
                int count = 0;
                sta = conn.createStatement();
                PreparedStatement psta = conn.prepareStatement("UPDATE CLASSES SET IS_ENABLED = ? WHERE CLASS_NAME = ?");
                psta.setBoolean(1, false);
                psta.setString(2, className);
                count = psta.executeUpdate();
                System.out.println(count + " record(s) updated.");

                psta = conn.prepareStatement("UPDATE CLASSES SET IS_ENABLED = ? WHERE CLASS_NAME = ? AND VERSION = ?");

                psta.setBoolean(1, true);
                psta.setString(2, className);
                psta.setString(3, version);

                count = psta.executeUpdate();

                System.out.println(count + " record(s) updated.");
                sta.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

}

输出

CLASSES Table created
2 record(s) created.
2 record(s) updated.
1 record(s) updated.
Version 1 Greet.getGreetMessage() : Hail to the King Baby!
2 record(s) updated.
1 record(s) updated.
Exception in thread "main" java.lang.LinkageError: loader (instance of  com/levent/classloader/DerbyServerClassLoader): attempted  duplicate class definition for name: "com/levent/greeter/Greeter"
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at com.levent.classloader.DerbyServerClassLoader.findClass(DerbyServerClassLoader.java:38)
    at com.levent.example.ClientClassLoaderDBVersionDemo.main(ClientClassLoaderDBVersionDemo.java:43)
4

2 回答 2

5

我怎么解决这个问题？有没有办法卸载一个类，或者无论如何重新加载一个同名的类？

没有办法强制1类卸载。除非旧类被卸载，否则无法将新版本的类加载到同一个类加载器中。（这是因为一个类的真实身份是一个包含类的完全限定名和类加载器身份的元组。）

解决方案是在新的类加载器中加载新版本的类。

目前尚不清楚这对您是否实用，但不幸的是，这是唯一可用的选择。JVM 以您无法破坏它的方式执行“重复类定义”检查。该检查具有安全性和 JVM 稳定性影响。

1 - 未被任何可访问对象引用的类最终将被 GC 卸载（取模 JVM 命令行选项、版本等）。但是，消除对类的所有引用可能会很棘手。此外，现在反复强制 GC 运行对整体性能不利。

于 2015-12-22T11:55:30.400 回答
1

我认为问题将来自ClassLoader您正在使用的父母。您没有重载 loadClass 方法，因此您在父类中委托，具体地在ClassLoader.getSystemClassLoader().

正如 javadoc 在https://docs.oracle.com/javase/7/docs/api/java/lang/ClassLoader.html#getSystemClassLoader()

此方法首先在运行时启动序列的早期调用，此时它创建系统类加载器并将其设置为调用线程的上下文类加载器。

您想加载更改后的类，但您正在委派Thread ClassLoader操作，这有点令人困惑。

你可以做这样的事情，使用你自己的类ClassLoader

package a;

import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;

public class ReloadTest {

    public static void main(String[] args) throws ClassNotFoundException, IOException {

        final Class<?> clazz = ReloadTest.class;

        System.out.println("Class: " +  clazz.hashCode());

        final URL[] urls = new URL[1];

        urls[0] =  clazz.getProtectionDomain().getCodeSource().getLocation();
        final ClassLoader delegateParent = clazz.getClassLoader().getParent();

        try (final URLClassLoader cl = new URLClassLoader(urls, delegateParent)) {

            final Class<?> reloadedClazz = cl.loadClass(clazz.getName());
            System.out.println("Class reloaded: " + reloadedClazz.hashCode());
            System.out.println("Are the same: " + (clazz != reloadedClazz) );
        }
    }
}

希望有所帮助！

PD：此链接与相同的问题有关，可能也有帮助Reload used classes at runtime Java

于 2015-12-22T11:58:53.273 回答