在代码中使用自定义的异常类，可以对异常进行统一的封装处理。使得整个项目的异常处理更规范、更统一、更优雅。同时，使得日志的记录上更加清晰，便于后续查日志定位问题。

　　以下为自定义异常类的完整过程：

1、需要自定义一个类，继承自系统的异常类。具体需要什么样类型的异常类，就继承自相应的系统类。

• 如果希望写一个检查性异常类，则需要继承 Exception 类。

如果你想写一个运行时异常类，那么需要继承 RuntimeException 类。

public class InvalidReqException extends Exception {    private final String userErrMsg;    private final ErrorCode errorCode;    public InvalidReqException(ErrorCode errorCode, String sysErrMsg, String userErrMsg) {        super(sysErrMsg);        this.userErrMsg = userErrMsg;        this.errorCode = errorCode;    }    public InvalidReqException(ErrorCode errorCode, String sysErrMsg, String userErrMsg, Throwable cause) {        super(sysErrMsg, cause);        this.userErrMsg = userErrMsg;        this.errorCode = errorCode;    }    public InvalidReqException(ErrorCode errorCode, String sysErrMsg) {        super(sysErrMsg);        this.userErrMsg = null;        this.errorCode = errorCode;    }    public ErrorCode getErrorCode() {        return errorCode;    }    public String getUserErrMsg() {        return userErrMsg;    }    public String getSysErrMsg() {        return getMessage();    }}

 

2、在需要抛出该异常的类或方法中，定义当出现异常时抛出自定义的异常。

static void checkTemplateValid(String content) throws InvalidReqException {        if (content.length() > 100 * 1024) {            throw new InvalidReqException(ErrorCode.TEMPLATE_INVALID, "template is too large, over 100KB",                    "模板HTML原文不得超过100KB");        }}　　

3、在使用到该类或方法的地方捕捉该异常(或继续抛出，交给上级去处理)，并对异常进行业务相关的响应与处理。

public void onMessage(Message message) {        String tenantId = (String) message.getMessageProperties().getHeaders().get(MQRouting.HEADER_TENANT_ID);        String requestId = (String) message.getMessageProperties().getHeaders().get(MQRouting.HEADER_REQ_ID);        String body = null;        CertApplyMsg msg;        try {            body = new String(message.getBody(), "utf8");            log.debug("received cert apply message: {}", body);            msg = JsonUtil.parse(body, CertApplyMsg.class);        } catch (IOException e) { // impossible to happen            log.error("message failed due to invalid format:" + body, e);            failMsg(MQRouting.EXCH_CERT, MQRouting.RK_CERT_APPLY_FAILED, message, e);            return;        }        LegalEntityCertBo entity = msg.getLegalEntityCert();        try {            if (entity.getCert().getStatus() != CertStatus.APPLYING) {                reconcileConcurrentUpdate(tenantId, requestId, msg); // due to retry                return;            }            LegalEntityCertBo created = certService.applyCert(entity.getOrganization(),                    entity.getCert().getCa(), entity.getLegalEntity());            if (!certService.updateCert(created, entity.getCert().getStatus())) { // due to data race                reconcileConcurrentUpdate(tenantId, requestId, msg);            } else {                enqueueCallback(tenantId, requestId, msg, created, null, null);                log.info("successfully processed cert application:" + body);            }        } catch (InvalidReqException e) {            log.error("applying cert rejected. body=" + body + ", message=" + message, e);            enqueueCallback(tenantId, requestId, msg, entity, e.getErrorCode(), e.getSysErrMsg());            rejectMsg(MQRouting.EXCH_CERT, MQRouting.RK_CERT_APPLY_REJECTED, message, e);        } catch (RuntimeException e) {            log.error("cert application failed, probably will retry, body=" + body + ", msg=" + e.getMessage(), e);            throw e;        }    }