使用Drools 7向导决策表执行Kie会话时触发空指针异常

我来帮你定位并解决这个Drools决策表引发的空指针异常（NPE）问题。从你的描述来看，普通DRL文件能正常运行，但KIE Workbench生成的决策表打包后就出问题，核心异常是ExceptionInInitializerError，这通常和类加载、类初始化或者规则中的类引用有关。

核心问题分析 #

ExceptionInInitializerError一般是类的静态代码块执行失败，或者静态变量初始化出错，结合你的场景，大概率是决策表生成的DRL中引用的实体类（InputDTO、CustomerDto）无法被正确加载或初始化，导致规则在初始化阶段抛出异常，进而引发后续的空指针问题。

具体修复步骤 #

1. 确保类包路径一致 #

首先检查决策表DRL的package声明和你的实体类实际包路径是否匹配。你的DRL开头是package gr.maven.test.testbuildinmaven;，如果InputDTO或CustomerDto不在这个包下，必须添加显式的导入语句：

package gr.maven.test.testbuildinmaven;
// 替换成你实际的包路径
import com.yourpackage.InputDTO;
import com.yourpackage.CustomerDto;

如果实体类确实在这个包下，也建议加上导入语句，避免类加载歧义。

2. 检查实体类的无参构造函数 #

Drools在通过new CustomerDto()实例化对象时，需要类有公共的无参构造函数。如果你的CustomerDto只定义了带参构造，一定要补充无参构造：

public class CustomerDto {
    // 必须添加无参构造
    public CustomerDto() {}
    
    // 其他属性和方法
    private String customerMessage;
    // getter和setter
}

3. 统一KieSession的类加载器 #

在REST服务环境中，类加载器隔离很容易导致问题。创建KieContainer时，指定使用当前线程的上下文类加载器，确保规则能加载到应用中的实体类：

KieServices kServices = KieServices.Factory.get();
// 使用上下文类加载器创建容器
KieContainer kContainer = kServices.getKieClasspathContainer(Thread.currentThread().getContextClassLoader());
StatelessKieSession kSession = kContainer.newStatelessKieSession();

4. 验证Jar包完整性 #

解压你打包的Jar文件，检查以下内容：

• 实体类（InputDTO.class、CustomerDto.class）是否存在
• 决策表生成的DRL文件是否在正确的包路径下
• 所有依赖的Drools相关类是否被正确打包（如果是fat jar的话）

你的代码与异常信息整理 #

Java执行代码 #

public CustomerDto executeStatelessPOC(final String param, final StatelessKieSession kSession ,final KieServices kServices) { 
    // Myexample is just the entry fact 
    InputDTO example = new InputDTO(); 
    example.setField1(param); 
    CustomerDto response = new CustomerDto(); 
    try { 
        KieCommands kieCommands = kServices.getCommands(); 
        List<Command> cmds = new ArrayList<Command>(); 
        cmds.add(kieCommands.newInsert(example)); 
        cmds.add(kieCommands.newInsert(response, "customerResponse")); 
        // Execute rules with facts provided before 
        ExecutionResults results = kSession.execute(kieCommands.newBatchExecution(cmds)); 
        // Get the response as part of the results 
        response = (CustomerDto) results.getValue("customerResponse"); 
        return response; 
    } catch (Exception e) { 
        System.out.println("Exception e: " + e.toString()); 
        System.out.println("Exception e: " + Arrays.toString(e.getStackTrace())); 
    } 
    return response; 
}

向导决策表生成的DRL源码 #

package gr.maven.test.testbuildinmaven; 

// from row number: 1 
rule "Row 1 TestDecisionTable" 
dialect "mvel" 
when 
    Input : InputDTO( field1 == "Test1" ) 
then 
    CustomerDto fact0 = new CustomerDto(); 
    fact0.setCustomerMessage( "yesss" ); 
    insertLogical( fact0 ); 
end

// from row number: 2 
rule "Row 2 TestDecisionTable" 
dialect "mvel" 
when 
    Input : InputDTO( field1 == "Test2" ) 
then 
    CustomerDto fact0 = new CustomerDto(); 
    fact0.setCustomerMessage( "yes2" ); 
    insertLogical( fact0 ); 
end

// from row number: 3 
rule "Row 3 TestDecisionTable" 
dialect "mvel" 
when 
    Input : InputDTO( field1 == "Test3" ) 
then 
    CustomerDto fact0 = new CustomerDto(); 
    fact0.setCustomerMessage( "yesss" ); 
    insertLogical( fact0 ); 
end

异常信息 #

WARNING: The following warnings have been detected: WARNING: Unknown HK2 failure detected: 

MultiException stack 1 of 2 
java.lang.ExceptionInInitializerError 
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) 
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) 
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) 
at java.lang.reflect.Constructor.newInstance(Constructor.java:423) 
at org.glassfish.hk2.utilities.reflection.ReflectionHelper.makeMe(ReflectionHelper.java:1129) 
at org.jvnet.hk2.internal.ClazzCreator.createMe(ClazzCreator.java:274) 
at org.jvnet.hk2.internal.ClazzCreator.create(ClazzCreator.java:368) 
at org.jvnet.hk2.internal.SystemDescriptor.create(SystemDescriptor.java:471) 
at org.glassfish.jersey.process.internal.RequestScope.findOrCreate(RequestScope.java:162) 
at org.jvnet.hk2.internal.Utilities.createService(Utilities.java:2064) 
at org.jvnet.hk2.internal.ServiceLocatorImpl.internalGetService(ServiceLocatorImpl.java:711) 
at org.jvnet.hk2.internal.ServiceLocatorImpl.getService(ServiceLocatorImpl.java:653) 
at org.glassfish.jersey.internal.inject.Injections.getOrCreate(Injections.java:169) 
at org.glassfish.jersey.server.model.MethodHandler$ClassBasedMethodHandler.getInstance(MethodHandler.java:185) 
at org.glassfish.jersey.server.internal.routing.PushMethodHandlerRouter.apply(PushMethodHandlerRouter.java:74) 
at org.glassfish.jersey.server.internal.routing.RoutingStage._apply(RoutingStage.java:109) 
at org.glassfish.jersey.server.internal.routing.RoutingStage._apply(RoutingStage.java:112) 
at org.glassfish.jersey.server.internal.routing.RoutingStage._apply(RoutingStage.java:112) 
at org.glassfish.jersey.server.internal.routing.RoutingStage._apply(RoutingStage.java:112) 
at org.glassfish.jersey.server.internal.routing.RoutingStage._apply(RoutingStage.java:112) 
at org.glassfish.jersey.server.internal.routing.RoutingStage.apply(RoutingStage.java:92) 
at org.glassfish.jersey.server.internal.routing.RoutingStage.apply(RoutingStage.java:61) 
at org.glassfish.jersey.process.internal.Stages.process(Stages.java:197) 
at org.glassfish.jersey.server.ServerRuntime$2.run(ServerRuntime.java:297) 
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:271) 
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:267) 
at org.glassfish.jersey.internal.Errors.process(Errors.java:315) 
at org.glassfish.jersey.internal.Errors.process(Errors.java:297) 
at org.glassfish.jersey.internal.Errors.process(Errors.java:267) 
at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:317) 
at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:288) 
at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:1110) 
at org.glassfish.jersey.grizz...

内容的提问来源于stack exchange，提问作者user3572380