解决方式：

1.配置文件插入

<bean id="processEngineConfiguration" class="org.activiti.spring.SpringProcessEngineConfiguration">

        <property name="dataSource" ref="dataSource" />

        <property name="databaseSchemaUpdate" value="true" />

        <property name="transactionManager" ref="transactionManager" />

        <!-- 中文乱码问题 -->

        <property name="activityFontName" value="宋体"></property>

        <property name="labelFontName" value="宋体"/>

    </bean>

2.生成图片代码中增加

public InputStream getDiagram(String processInstanceId) {

        // 查询流程实例

        ProcessInstance pi = this.runtimeService.createProcessInstanceQuery()

                .processInstanceId(processInstanceId).singleResult();

        // 强制类型转换为RepositoryServiceImpl

        RepositoryServiceImpl rs = (RepositoryServiceImpl) this.processEngine

                .getRepositoryService();

        //1使用Spring注入

        ProcessEngineConfigurationImpl processEngineConfigurationImpl = ((ProcessEngineImpl) this.processEngine)

                .getProcessEngineConfiguration();

        Context.setProcessEngineConfiguration(processEngineConfigurationImpl);

       //2.不使用Spring注入

        // ProcessEngineImpl defaultProcessEngine = (ProcessEngineImpl) ProcessEngines

        // .getDefaultProcessEngine();

        // Context.setProcessEngineConfiguration(defaultProcessEngine

        // .getProcessEngineConfiguration());

        BpmnModel bpmnModel = repositoryService.getBpmnModel(pi

                .getProcessDefinitionId());

        // 转换为输入流

        InputStream is = ProcessDiagramGenerator.generateDiagram(bpmnModel,

                "png", runtimeService.getActiveActivityIds(processInstanceId));

        return is;

    }