给出详细的java 代码

以下是一个使用Kettle将两个MySQL库中的表数据合并到Excel文件中的Java代码示例，包括了连接MySQL数据库、读取表数据、合并数据、写入Excel文件等基本步骤：

import org.pentaho.di.core.KettleEnvironment;
import org.pentaho.di.core.database.DatabaseMeta;
import org.pentaho.di.core.exception.KettleException;
import org.pentaho.di.core.plugins.PluginRegistry;
import org.pentaho.di.core.plugins.StepPluginType;
import org.pentaho.di.core.row.RowMetaInterface;
import org.pentaho.di.core.row.ValueMetaInterface;
import org.pentaho.di.core.row.value.ValueMetaString;
import org.pentaho.di.trans.Trans;
import org.pentaho.di.trans.TransMeta;
import org.pentaho.di.trans.step.StepMeta;
import org.pentaho.di.trans.step.StepMetaInterface;
import org.pentaho.di.trans.step.StepTypeInterface;
import org.pentaho.di.trans.steps.exceloutput.ExcelOutputMeta;
import org.pentaho.di.trans.steps.exceloutput.ExcelOutputStepMeta;
import org.pentaho.di.trans.steps.mergejoin.MergeJoinMeta;
import org.pentaho.di.trans.steps.mysqlinput.MySQLInputMeta;

import java.util.ArrayList;
import java.util.List;

public class KettleExample {
    public static void main(String[] args) throws KettleException {
        // 初始化Kettle环境
        KettleEnvironment.init();
        
        // 创建转换元数据对象
        TransMeta transMeta = new TransMeta();
        
        // 创建MySQL数据库连接信息
        DatabaseMeta dbMeta1 = new DatabaseMeta("db1", "MYSQL", "Native", "localhost", "db1", "3306", "username", "password");
        DatabaseMeta dbMeta2 = new DatabaseMeta("db2", "MYSQL", "Native", "localhost", "db2", "3306", "username", "password");
        
        // 添加MySQL输入步骤1
        MySQLInputMeta inputMeta1 = new MySQLInputMeta();
        inputMeta1.setDatabaseMeta(dbMeta1);
        inputMeta1.setSQL("SELECT * FROM table1");
        StepMeta inputStep1 = new StepMeta("MySQL Input 1", inputMeta1);
        transMeta.addStep(inputStep1);
        
        // 添加MySQL输入步骤2
        MySQLInputMeta inputMeta2 = new MySQLInputMeta();
        inputMeta2.setDatabaseMeta(dbMeta2);
        inputMeta2.setSQL("SELECT * FROM table2");
        StepMeta inputStep2 = new StepMeta("MySQL Input 2", inputMeta2);
        transMeta.addStep(inputStep2);
        
        // 添加合并步骤
        MergeJoinMeta mergeMeta = new MergeJoinMeta();
        mergeMeta.setJoinType("FULL OUTER JOIN");
        mergeMeta.setKeyFields1(new String[]{"id"});
        mergeMeta.setKeyFields2(new String[]{"id"});
        StepMeta mergeStep = new StepMeta("Merge Join", mergeMeta);
        transMeta.addStep(mergeStep);
        transMeta.addTransHop(new TransHopMeta(inputStep1, mergeStep));
        transMeta.addTransHop(new TransHopMeta(inputStep2, mergeStep));
        
        // 添加Excel输出步骤
        ExcelOutputMeta outputMeta = new ExcelOutputMeta();
        outputMeta.setFileName("output.xlsx");
        outputMeta.setSheetname("Sheet1");
        outputMeta.setHeaderEnabled(true);
        List<ExcelOutputStepMeta.FieldMapping> fields = new ArrayList<>();
        RowMetaInterface outputFields = new RowMetaInterface() {
            @Override
            public int size() {
                return 3;
            }

            @Override
            public ValueMetaInterface getValueMeta(int index) {
                switch (index) {
                    case 0:
                        return new ValueMetaString("id");
                    case 1:
                        return new ValueMetaString("name");
                    case 2:
                        return new ValueMetaString("value");
                    default:
                        return null;
                }
            }

            @Override
            public String[] getFieldNames() {
                return new String[]{"id", "name", "value"};
            }

            @Override
            public int indexOfValue(String valueName) {
                switch (valueName) {
                    case "id":
                        return 0;
                    case "name":
                        return 1;
                    case "value":
                        return 2;
                    default:
                        return -1;
                }
            }
        };
        fields.add(new ExcelOutputStepMeta.FieldMapping(0, "id"));
        fields.add(new ExcelOutputStepMeta.FieldMapping(1, "name"));
        fields.add(new ExcelOutputStepMeta.FieldMapping(2, "value"));
        outputMeta.setFieldMappings(fields);
        outputMeta.setOutputFields(outputFields);
        StepMeta outputStep = new StepMeta("Excel Output", outputMeta);
        transMeta.addStep(outputStep);
        transMeta.addTransHop(new TransHopMeta(mergeStep, outputStep));
        
        // 运行转换
        Trans trans = new Trans(transMeta);
        trans.execute(null);
        trans.waitUntilFinished();
        if (trans.getErrors() > 0) {
            System.out.println("Error");
        } else {
            System.out.println("Success");
        }
    }
}

需要注意的是，这只是一个简单的示例代码，实际应用中需要根据具体的数据表结构和业务需求进行相应的修改和调整。