问题描述
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.将类文件加载到内存数据库( Apache Derby Db )中，然后类加载器从检索 .class 文件. BLOB 列.
I have a working code that loads dynamically different Class Implementations with different Class Names. The class files are loaded into the in-memory database (Apache Derby Db), and classloader retrieve the .class file from the BLOB columns.
我要做的是，将 .class 文件作为带有版本列和IS_ENABLED标志的二进制BLOB插入，然后classloader将在运行时加载不同版本的类.将有与已编译类版本相同数量的数据库条目，并且只有一个将IS_ENABLED标志设置为 TRUE 的类.
What I want to do is, inserting the .class files as binary BLOB with version column and IS_ENABLED flags, then classloader will load the class for different versions on run-time. There will be db entries same amount of the compiled class versions and there will only one class with IS_ENABLED flag set to TRUE.
因为我尝试使用自定义类加载器加载相同的类名称，所以出现以下异常；
Because that I try to load the same class name with custom classloader, I get the following Exception;
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)
有两个不同的 .class 文件( Greeter.class.v1 ， Greeter.class.v2 )(在代码)( Greeter.java )
There are two different .class files (Greeter.class.v1, Greeter.class.v2) (inserted at the begining of the code) for the same Interface (Greeter.java)
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db中，然后，数据库依次检索.class文件并进行加载.加载具有相同名称的类时，会发生异常.
In the beginning of test code, class files are retrieved from the lib/classes/ folder and inserted as blob binary data to the in-memory db, after then, .class files are sequentially retrieved by the database and loaded. While loading the class with same name, Exception occurs.
我该如何解决这个问题?有什么方法可以卸载一个类，或者是否可以以相同的名称重新加载一个类?
How can I solve this problem? Is there any way to unload a class, or else, anyway to reload a class with same name?
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-Greeter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)
推荐答案
我认为问题可能出在您使用的父级ClassLoader上.您没有重载loadClass方法，因此在父类中(具体在ClassLoader.getSystemClassLoader()中)进行委派.
I think the problem would become from the parent ClassLoader you are using. You are not overloading loadClass method, so you are delegating in parent class, concretely in ClassLoader.getSystemClassLoader().
javadoc在 https上说://docs.oracle.com/javase/7/docs/api/java/lang/ClassLoader.html#getSystemClassLoader()
您想加载更改后的类，但是将操作委托给Thread ClassLoader，这有点令人困惑.
You want to load the changed classes but you are delegating to the Thread ClassLoader the operation, is a bit confusing.
您可以使用自己的Class ClassLoader
You can do something like this, using your own Class 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) );
}
}
}
希望有帮助！
PD:此链接与相同的问题有关，可能也有帮助.使用重载Java运行时的类
P.D: This link is related to the same problems, may it help too Reload used classes at runtime Java
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