Java d有一个空列表，不知道任何异常（日志中除外）。（或null但我最近尝试使用exception而不是null） public List<Product> doSelectAvailableProducts( String userid){

d有一个空列表，不知道任何异常（日志中除外）。（或

null

但我最近尝试使用exception而不是null）

public List<Product> doSelectAvailableProducts( String userid){
    Connection con = null; 
    Statement stmt = null; 
    ResultSet rs = null; 
    List<Product> cl = new ArrayList<Product>(); 
    try{ 
        con = datasource.getConnection(); 
    } 
    catch( SQLException e){ 
        e.printStackTrace(); 
    } 
  try{ 
      stmt = con.createStatement(); 
      String sql = "select * from Product where id not in " +"(select cid from registration where uid ='" + userid + "')";
      rs = stmt.executeQuery(sql); 
      while( rs.next() ){ 
        Product product = new Product(); 
        product.setId(rs.getInt("id")); 
        product.setName(rs.getString("name"));
        product.setDescription( rs.getString("description")); 
        product.setPrice( Double.parseDouble(rs.getString("price"))); cl.add(product); } 
      return cl; 
  } 
  catch( SQLException e ){ 
      e.printStackTrace(); 
  } 
  finally { 
      if(stmt != null) { 
          try { stmt.close();} 
          catch (Exception e) { e.printStackTrace(); } 
      } 
      if(con != null) { 
          try { con.close();}
          catch (Exception e) { e.printStackTrace(); }        
      }}  }

catch( SQLException e ){ 
    e.printStackTrace(); 
    return new ArrayList<Product>();     // <--
}

finally { 
      if(stmt != null) { 
          try { stmt.close();} 
          catch (Exception e) { e.printStackTrace(); } 
      } 
      if(con != null) { 
          try { con.close();}
          catch (Exception e) { e.printStackTrace(); } 

      }

    }  
      return null;

}

  }

List<Product> doSelectAvailableProducts result=object.doSelectAvailableProducts(aStringId);
if(result!=null){
// Do you next work flow here...
}

public List<Product> doSelectAvailableProducts( String userid){
    Connection con = null; 
    Statement stmt = null; 
    ResultSet rs = null; 
    List<Product> cl = new ArrayList<Product>(); 

    try{ 
        con = datasource.getConnection(); 
    } 
    catch( SQLException e){ 
        e.printStackTrace();
        return cl;
    } 
  try{ 
      stmt = con.createStatement(); 
      String sql = "select * from Product where id not in " +"(select cid from registration where uid ='" + userid + "')";
      rs = stmt.executeQuery(sql); 
      while( rs.next() ){ 
        Product product = new Product(); 
        product.setId(rs.getInt("id")); 
        product.setName(rs.getString("name"));
        product.setDescription( rs.getString("description")); 
        product.setPrice( Double.parseDouble(rs.getString("price"))); cl.add(product); } 
        return cl; 
  } 
  catch( SQLException e ){ 
      e.printStackTrace();
      return cl;
  } 
  finally { 
      if(stmt != null) { 
          try { stmt.close();} 
          catch (Exception e) { e.printStackTrace(); } 
      } 
      if(con != null) { 
          try { con.close();}
          catch (Exception e) { e.printStackTrace(); }        
      }}  return cl;}

public List<Product> doSelectAvailableProducts( String userid){

  Connection con = null; 
  Statement stmt = null; 
  ResultSet rs = null; 
  List<Product> cl = new ArrayList<Product>(); 

  try{ 

    con = datasource.getConnection(); 

  } 
  catch( SQLException e){

    //throw this exception or add a return here?
    e.printStackTrace(); 
  } 

  try{ 

    stmt = con.createStatement(); 
    String sql = "select * from Product where id not in " +"(select cid from registration where uid ='" + userid + "')";

    //Exception might be thrown here, so the return statment below isn't reached
    //then all the code in the catch and finally run, but nothing is returned
    rs = stmt.executeQuery(sql);

    while( rs.next() ){ 
    Product product = new Product(); 
    product.setId(rs.getInt("id")); 
    product.setName(rs.getString("name"));
    product.setDescription( rs.getString("description")); 
    product.setPrice( Double.parseDouble(rs.getString("price"))); cl.add(product); }

    // Last return statement, never reached
    return cl;  

  } 
  catch( SQLException e ){ 

    //throw this exception or add a return here?
    e.printStackTrace();
  } 
  finally { 

    if(stmt != null) { 

      try {
        stmt.close();
      } 
      catch (Exception e) {
        e.printStackTrace(); 
      } 
    } 

    if(con != null) { 

      try {
        con.close();
      }
      catch (Exception e) {
        e.printStackTrace();
      }
    }

    //add a return here?
  }

  //add a return here?
}