首先，您需要确保您的AWS Lambda函数在与RDS Postgres数据库相同的VPC中运行。然后，在您的Java Lambda函数中，您需要使用AWS提供的Java SDK来连接到Postgres数据库。以下是一些示例代码以获取连接：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class PostgresDBConnection {
    // RDS Postgres Database Details
    private static final String dbURL = "jdbc:postgresql://:/";
    private static final String masterUsername = "";
    private static final String masterUserPassword = "";
    
    public static Connection getConnection() throws SQLException {
        return DriverManager.getConnection(dbURL, masterUsername, masterUserPassword);
    }
}

上面的示例代码使用JDBC驱动程序从Java应用程序连接到RDS Postgres数据库。您需要将HOST，PORT，DATABASE_NAME，MASTER_USERNAME和MASTER_USER_PASSWORD替换为实际的值，例如：

private static final String dbURL = "jdbc:postgresql://my-rds-instance.us-west-2.rds.amazonaws.com:5432/mydatabase";
private static final String masterUsername = "myuser";
private static final String masterUserPassword = "mypassword";

然后，在AWS Lambda函数中，您可以使用以下代码获取Postgres数据库连接：

import java.sql.Connection;
import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestHandler;

public class MyLambdaFunction implements RequestHandler {

    @Override
    public String handleRequest(String input, Context context) {
        Connection conn = null;

        try {
            conn = PostgresDBConnection.getConnection();
            // use the connection to interact with the database
        } catch (Exception e) {
            context.getLogger().log("Exception: " + e.getMessage());
        } finally {
            if (conn != null) {
                try {
                    conn.close();
                } catch (SQLException e) {
                    context.getLogger().log("Exception: " + e.getMessage());
                }
            }
        }

        return "success";
    }
}

上面的示例代码