我似乎遇到了我认为使用 db4o 和 NetBeans 的 ClassLoader 问题。当我使用来自 .../jre/lib/ext 的相同 jar 文件从终端运行完全相同的代码时，一切正常。问题是，当我对在运行时使用 ClassLoader 加载的某些类进行本机查询时，我从数据库中得到一个空列表，我肯定应该在其中得到一个包含一些元素的列表（正如我所说，相同的代码有效从命令行很好）。我觉得这可能是因为 NetBeans ClassLoader 与 JVM ClassLoader 的工作方式不同，但我不知道，我当然不是这方面的专家。这是我的主要功能的代码.....
////////////////////////////////////////////////////////////////////////////////////////////
package gql;
import java.io.*;
import java.util.*;
import com.db4o.*;
public class GQL {
////////////////////////////////////////////////////////////////////////////
// GLOBAL VARIABLES
////////////////////////////////////////////////////////////////////////////
private static ObjectSet dbObjects;
private static LinkedList classes = new LinkedList();
private static String dbPath, classPath;
private static ObjectContainer db;
private static ClassLoader coreClassLoader =
ClassLoader.getSystemClassLoader();
private static ClassLoader subClassLoader =
ClassLoader.getSystemClassLoader();
public static void main(String[] args) {
////////////////////////////////////////////////////////////////////////
// CREATE DATABASE OBJECT
////////////////////////////////////////////////////////////////////////
// If no path to a database is provided on the command line, print
// error and exit program
if (args.length < 1) {
System.err.println("\nError: no database path provided.\n");
return;
} else if (args.length > 1) {
dbPath = args[0];
// TODO - dubug command line classpath
classPath = args[1];
db = Db4o.openFile(dbPath);
} else { // We assume that the database Classes are stored somewhere
dbPath = args[0]; // along the CLASSPATH, and therefore classPath
classPath = ""; // can be left empty
db = Db4o.openFile(dbPath);
}
System.out.print("GQL> ");
// The prompt of the interpreter is within a do-while loop, which can
// be terminated by entering "exit"
do {
try {
/////////////////////////////////////////////////////////////////
// READ IN QUERY FILE
/////////////////////////////////////////////////////////////////
// We create a Scanner object to read tokens from the standard in
// stream - these will be our DLOG files provided by the user
Scanner fileScanner = new Scanner(System.in);
String GQLFile = fileScanner.next();
// Break loop and exit program if user enters "exit"
if (GQLFile.equalsIgnoreCase("exit")) {
break;
// If the user input is not preceeded by "@" and teminated with
// ";" then the input is invalid - the user is prompted again
} else if (!(GQLFile.substring(0,1).equals("@")) ||
!(GQLFile.substring(GQLFile.length()-1,
GQLFile.length()).equals(";"))) {
System.out.println("\nInvalid input.\nUsage: "
+ " @filename;\n");
System.out.print("GQL> ");
continue;
} else {
// Parse out the "@" and ";" from the user's input and send
// this to a file Reader object
GQLFile = GQLFile.substring(1,GQLFile.length()-1);
}
// Now we create a reader object and give it the user's parsed
// input - in the event of a FileNotFoundException, the user is
// prompted again
Reader reader;
try {
reader = new FileReader(GQLFile);
} catch (FileNotFoundException e) {
System.out.println("\nFile " + GQLFile +
" does not exist.\n");
System.out.print("GQL> ");
continue;
}
/////////////////////////////////////////////////////////////////
// PARSE QUERY
/////////////////////////////////////////////////////////////////
// The parser and Lexer objects are created in the parser.java
// and Lexer.java files, respectively - The parser takes the
// Lexer as an argument - the value variable generated by the
// parse() method will return the topmost grammar construct,
// which in this case is a Query object
parser p = new parser(new Lexer(reader));
Query query = (Query) p.parse().value;
/////////////////////////////////////////////////////////////////
System.out.println("\n----------------------------Input Query-----" +
"-----------------------\n");
System.out.println("\n SUCCESSFUL PARSE " +
" \n");
System.out.println("--------------------------------------" +
"------------------------------\n");
/////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////
// LOAD ALL CLASSES USED IN DATABASE INTO RUNTIME
/////////////////////////////////////////////////////////////////
// databse Classes should be kept on the CLASSPATH, or the path
// to these classes should be provided as a second command line
// argument
boolean coreClassesLoaded = loadCoreClasses(coreClassLoader,
classPath);
if (!coreClassesLoaded) {
System.err.println("\nError: one or more of core Classes"
+ "Node, Egge and SimplePath could not be found.\n");
db.close();
return;
}
//
System.out.println("Core classes loaded.\n");
boolean subclassesLoaded = query.loadSubclasses(subClassLoader,
classPath);
if (!subclassesLoaded) {
System.err.println("\nError: subclasses could not be" +
" loaded.\n");
db.close();
return;
}
//
System.out.println("Subclasses loaded.\n");
/////////////////////////////////////////////////////////////////
// MAKE SURE THE DATABASE ACTUALLY CONTAINS SOME OBJECTS AND,
// IF SO, PUT AN INSTANCE OF EACH CLASS REPRESENTED INTO THE
// LINKEDLIST CLASSLIST - SINCE WE LOADED THE DATABASE CLASSES
// INTO THE RUNTIME ENVIRONMENT, OBJECTS RETURNED BY DATABASE
// QUERIES WILL REMAIN TRUE TO THEIR TYPE; IF WE HADN'T DONE
// THIS, THESE OBJECTS WOULD BE RETURNED AS TYPE GENERICOBJECT
/////////////////////////////////////////////////////////////////
dbObjects = db.queryByExample(Object.class);
if (dbObjects.hasNext()) {
query.addClassesToList(dbObjects, classes);
} else {
System.err.println("\nError: no objects in database.\n");
db.close();
return;
}
//
System.out.println(classes);
/////////////////////////////////////////////////////////////////
// SEMANTIC CHECKS //
/////////////////////////////////////////////////////////////////
boolean headArgsAreSet = query.setHeadArgClasses(classes);
if (!headArgsAreSet) {
db.close();
return;
}
boolean typesMatch = query.checkQueryTypes(db);
if (!typesMatch) {
db.close();
return;
}
/////////////////////////////////////////////////////////////////
// EVALUATION
/////////////////////////////////////////////////////////////////
query.evaluateQuery(db);
} catch (Exception e) {
System.out.println("\nSYNTAX ERROR\n");
e.printStackTrace();
}
System.out.print("GQL> ");
} while (true);
System.out.println("\nExiting...\n");
db.close();
}
private static boolean loadCoreClasses(ClassLoader coreClassLoader,
String classPath) {
try {
coreClassLoader.loadClass("Node");
coreClassLoader.loadClass("Edge");
coreClassLoader.loadClass("SimplePath");
} catch (ClassNotFoundException cnfe) {
return false;
}
return true;
}
}
////////////////////////////////////////////////////////////////////////////////////////////
奇怪的是，我需要的类肯定会加载到运行时环境中，因为我使用它们来设置一些类成员变量，例如在“语义检查”部分中。所以就像应用程序可以看到动态加载的类，但 db4o API/数据库不能。此外，我将 Class jar 和 db4o jar 设置为 Netbeans 库，而不仅仅是在 .../jre/lib/ext 中。这是 Class 中的代码片段，我在其中实际使用了给我带来问题的 db4o 本机查询...
////////////////////////////////////////////////////////////////////////////////////////////
public void evaluateQuery(ObjectContainer db) {
if (this.hasPredicate) {
;
} else {
if (this.isNode) {
List nodes = db.query(new Predicate() {
@Override
public boolean match(Node node) {
return (node.getName().equals("5"));
}
});
System.out.println("\n_________________RESULT__________________________");
System.out.println("\nNode: " + nodes.get(0).getName()
//+ ".\n");
}
}
}
////////////////////////////////////////////////////////////////////////////////////////////
...如果我执行以下操作，我仍然会得到一个空列表...
////////////////////////////////////////////////////////////////////////////////////////////
public void evaluateQuery(ObjectContainer db) {
if (this.hasPredicate) {
;
} else {
if (this.isNode) {
List nodes = db.queryByExample(Node.class);
System.out.println(nodes.size());
for (int i = 0; i < nodes.size(); i++) {
System.out.println("\nNode: " + nodes.get(i).getName());
}
}
}
}
////////////////////////////////////////////////////////////////////////////////////////////