1.注解

在自定义上面不加元注解，那么它将几乎毫无作用

2.常用的元注解

2.1@Target

@Target注解，是专门用来限定某个自定义注解能够被应用在哪些Java元素上面的。它使用一个枚举类型定义如下

TYPE,

    /** 属性的声明 */
    FIELD,

    /** 方法的声明 */
    METHOD,

    /** 方法形式参数声明 */
    PARAMETER,

    /** 构造方法的声明 */
    CONSTRUCTOR,

    /** 局部变量声明 */
    LOCAL_VARIABLE,

    /** 注解类型声明 */
    ANNOTATION_TYPE,

    /** 包的声明 */
    PACKAGE

2.2@Retention

@Retention注解，翻译为持久力、保持力。即用来修饰自定义注解的生命力。 注解的生命周期有三个阶段：1、Java源文件阶段；2、编译到class文件阶段；3、运行期阶段。同样使用了RetentionPolicy枚举类型定义了三个阶段：

SOURCE,

    /**
     * Annotations are to be recorded in the class file by the compiler
     * but need not be retained by the VM at run time.  This is the default
     * behavior.
     * （注解将被编译器记录在class文件中，但在运行时不会被虚拟机保留，这是一个默认的行为）
     */
    CLASS,

    /**
     * Annotations are to be recorded in the class file by the compiler and
     * retained by the VM at run time, so they may be read reflectively.
     * （注解将被编译器记录在class文件中，而且在运行时会被虚拟机保留，因此它们能通过反射被读取到）
     * @see java.lang.reflect.AnnotatedElement
     */
    RUNTIME

3.自定义注解ContentType代码 + BaseController代码：

ContentType

@Retention(RetentionPolicy.RUNTIME)  //被反射读取到  class
@Target(ElementType.METHOD)   //应用到方法上

public @interface ContentType {
    String  value();
    int id() default 1;
    String name() default  "Ellie";
}

BaseController

if (method.isAnnotationPresent(ContentType.class)){
                ContentType contentType = method.getAnnotation(ContentType.class);
                String type = contentType.value();//image/jpeg  application/json;charset=utf-8

                resp.setContentType(type);
            }else{
                resp.setContentType("text/html;charset=utf-8");
            }

3.1response.setContentType()的作用及参数

**response.setContentType(MIME)的作用是使客户端浏览器，区分不同种类的数据，并根据不同的MIME调用浏览器内不同的程序嵌入模块来处理相应的数据。 **

例如web浏览器就是
通过MIME类型来判断文件是GIF图片。
通过MIME类型来处理json字符串

这 个方法设置发送到客户端的响应的内容类型，此时响应还没有提交。给出的内容类型可以包括字符编码说明

例 如：text/html;charset=UTF-8.
如果该方法在getWriter()方法被调用之前调用，
那么响应的字符编码将仅从给出的内容类型 中设置。

如果在getWriter()方法被调用之后或者在被提交之后调用，
将不会设置响应的字符编码.

在使用http协议的情况中，该方法设 置 Content-type实体报头
一般在Servlet中，习惯性的会首先设置请求以及响应的内容类型以及编码方式：
response.setContentType(“text/html;charset=UTF-8”);
request.setCharacterEncoding(“UTF-8”);

3.2下面是其他地方找到的：response.setContentType()的String参数及对应类型

文件扩展名 Content-Type(Mime-Type) 文件扩展名 Content-Type(Mime-Type) .（ 二进制流，不知道下载文件类型） application/octet-stream .tif image/tiff .001 application/x-001 .301 application/x-301 .323 text/h323 .906 application/x-906 .907 drawing/907 .a11 application/x-a11 .acp audio/x-mei-aac .ai application/postscript .aif audio/aiff .aifc audio/aiff .aiff audio/aiff .anv application/x-anv .asa text/asa .asf video/x-ms-asf .asp text/asp .asx video/x-ms-asf .au audio/basic .avi video/avi .awf application/vnd.adobe.workflow .biz text/xml .bmp application/x-bmp .bot application/x-bot .c4t application/x-c4t .c90 application/x-c90 .cal application/x-cals .cat application/vnd.ms-pki.seccat .cdf application/x-netcdf .cdr application/x-cdr .cel application/x-cel .cer application/x-x509-ca-cert .cg4 application/x-g4 .cgm application/x-cgm .cit application/x-cit .class java/ .cml text/xml .cmp application/x-cmp .cmx application/x-cmx .cot application/x-cot .crl application/pkix-crl .crt application/x-x509-ca-cert .csi application/x-csi .css text/css .cut application/x-cut .dbf application/x-dbf .dbm application/x-dbm .dbx application/x-dbx .dcd text/xml .dcx application/x-dcx .der application/x-x509-ca-cert .dgn application/x-dgn .dib application/x-dib .dll application/x-msdownload .doc application/msword .dot application/msword .drw application/x-drw .dtd text/xml .dwf Model/vnd.dwf .dwf application/x-dwf .dwg application/x-dwg .dxb application/x-dxb .dxf application/x-dxf .edn application/vnd.adobe.edn .emf application/x-emf .eml message/rfc822 .ent text/xml .epi application/x-epi .eps application/x-ps .eps application/postscript .etd application/x-ebx .exe application/x-msdownload .fax image/fax .fdf application/vnd.fdf .fif application/fractals .fo text/xml .frm application/x-frm .g4 application/x-g4 .gbr application/x-gbr . application/x- .gif image/gif .gl2 application/x-gl2 .gp4 application/x-gp4 .hgl application/x-hgl .hmr application/x-hmr .hpg application/x-hpgl .hpl application/x-hpl .hqx application/mac-binhex40 .hrf application/x-hrf .hta application/hta .htc text/x-component .htm text/html .html text/html .htt text/webviewhtml .htx text/html .icb application/x-icb .ico image/x-icon .ico application/x-ico .iff application/x-iff .ig4 application/x-g4 .igs application/x-igs .iii application/x-iphone .img application/x-img .ins application/x-internet-signup .isp application/x-internet-signup .IVF video/x-ivf .java java/* .jfif image/jpeg .jpe image/jpeg .jpe application/x-jpe .jpeg image/jpeg .jpg image/jpeg .jpg application/x-jpg .js application/x-javascript .jsp text/html .la1 audio/x-liquid-file .lar application/x-laplayer-reg .latex application/x-latex .lavs audio/x-liquid-secure .lbm application/x-lbm .lmsff audio/x-la-lms .ls application/x-javascript .ltr application/x-ltr .m1v video/x-mpeg .m2v video/x-mpeg .m3u audio/mpegurl .m4e video/mpeg4 .mac application/x-mac .man application/x-troff-man .math text/xml .mdb application/msaccess .mdb application/x-mdb .mfp application/x-shockwave-flash .mht message/rfc822 .mhtml message/rfc822 .mi application/x-mi .mid audio/mid .midi audio/mid .mil application/x-mil .mml text/xml .mnd audio/x-musicnet-download .mns audio/x-musicnet-stream .mocha application/x-javascript .movie video/x-sgi-movie .mp1 audio/mp1 .mp2 audio/mp2 .mp2v video/mpeg .mp3 audio/mp3 .mp4 video/mpeg4 .mpa video/x-mpg .mpd application/vnd.ms-project .mpe video/x-mpeg .mpeg video/mpg .mpg video/mpg .mpga audio/rn-mpeg .mpp application/vnd.ms-project .mps video/x-mpeg .mpt application/vnd.ms-project .mpv video/mpg .mpv2 video/mpeg .mpw application/vnd.ms-project .mpx application/vnd.ms-project .mtx text/xml .mxp application/x-mmxp .net image/pnetvue .nrf application/x-nrf .nws message/rfc822 .odc text/x-ms-odc .out application/x-out .p10 application/pkcs10 .p12 application/x-pkcs12 .p7b application/x-pkcs7-certificates .p7c application/pkcs7-mime .p7m application/pkcs7-mime .p7r application/x-pkcs7-certreqresp .p7s application/pkcs7-signature .pc5 application/x-pc5 .pci application/x-pci .pcl application/x-pcl .pcx application/x-pcx .pdf application/pdf .pdf application/pdf .pdx application/vnd.adobe.pdx .pfx application/x-pkcs12 .pgl application/x-pgl .pic application/x-pic .pko application/vnd.ms-pki.pko .pl application/x-perl .plg text/html .pls audio/scpls .plt application/x-plt .png image/png .png application/x-png .pot application/vnd.ms-powerpoint .ppa application/vnd.ms-powerpoint .ppm application/x-ppm .pps application/vnd.ms-powerpoint .ppt application/vnd.ms-powerpoint .ppt application/x-ppt .pr application/x-pr .prf application/pics-rules .prn application/x-prn .prt application/x-prt .ps application/x-ps .ps application/postscript .ptn application/x-ptn .pwz application/vnd.ms-powerpoint .r3t text/vnd.rn-realtext3d .ra audio/vnd.rn-realaudio .ram audio/x-pn-realaudio .ras application/x-ras .rat application/rat-file .rdf text/xml .rec application/vnd.rn-recording .red application/x-red .rgb application/x-rgb .rjs application/vnd.rn-realsystem-rjs .rjt application/vnd.rn-realsystem-rjt .rlc application/x-rlc .rle application/x-rle .rm application/vnd.rn-realmedia .rmf application/vnd.adobe.rmf .rmi audio/mid .rmj application/vnd.rn-realsystem-rmj .rmm audio/x-pn-realaudio .rmp application/vnd.rn-rn_music_package .rms application/vnd.rn-realmedia-secure .rmvb application/vnd.rn-realmedia-vbr .rmx application/vnd.rn-realsystem-rmx .rnx application/vnd.rn-realplayer .rp image/vnd.rn-realpix .rpm audio/x-pn-realaudio-plugin .rsml application/vnd.rn-rsml .rt text/vnd.rn-realtext .rtf application/msword .rtf application/x-rtf .rv video/vnd.rn-realvideo .sam application/x-sam .sat application/x-sat .sdp application/sdp .sdw application/x-sdw .sit application/x-stuffit .slb application/x-slb .sld application/x-sld .slk drawing/x-slk .smi application/smil .smil application/smil .smk application/x-smk .snd audio/basic .sol text/plain .sor text/plain .spc application/x-pkcs7-certificates .spl application/futuresplash .spp text/xml .ssm application/streamingmedia .sst application/vnd.ms-pki.certstore .stl application/vnd.ms-pki.stl .stm text/html .sty application/x-sty .svg text/xml .swf application/x-shockwave-flash .tdf application/x-tdf .tg4 application/x-tg4 .tga application/x-tga .tif image/tiff .tif application/x-tif .tiff image/tiff .tld text/xml .top drawing/x-top .torrent application/x-bittorrent .tsd text/xml .txt text/plain .uin application/x-icq .uls text/iuls .vcf text/x-vcard .vda application/x-vda .vdx application/vnd.visio .vml text/xml .vpg application/x-vpeg005 .vsd application/vnd.visio .vsd application/x-vsd .vss application/vnd.visio .vst application/vnd.visio .vst application/x-vst .vsw application/vnd.visio .vsx application/vnd.visio .vtx application/vnd.visio .vxml text/xml .wav audio/wav .wax audio/x-ms-wax .wb1 application/x-wb1 .wb2 application/x-wb2 .wb3 application/x-wb3 .wbmp image/vnd.wap.wbmp .wiz application/msword .wk3 application/x-wk3 .wk4 application/x-wk4 .wkq application/x-wkq .wks application/x-wks .wm video/x-ms-wm .wma audio/x-ms-wma .wmd application/x-ms-wmd .wmf application/x-wmf .wml text/vnd.wap.wml .wmv video/x-ms-wmv .wmx video/x-ms-wmx .wmz application/x-ms-wmz .wp6 application/x-wp6 .wpd application/x-wpd .wpg application/x-wpg .wpl application/vnd.ms-wpl .wq1 application/x-wq1 .wr1 application/x-wr1 .wri application/x-wri .wrk application/x-wrk .ws application/x-ws .ws2 application/x-ws .wsc text/scriptlet .wsdl text/xml .wvx video/x-ms-wvx .xdp application/vnd.adobe.xdp .xdr text/xml .xfd application/vnd.adobe.xfd .xfdf application/vnd.adobe.xfdf .xhtml text/html .xls application/vnd.ms-excel .xls application/x-xls .xlw application/x-xlw .xml text/xml .xpl audio/scpls .xq text/xml .xql text/xml .xquery text/xml .xsd text/xml .xsl text/xml .xslt text/xml .xwd application/x-xwd .x_b application/x-x_b .sis application/vnd.symbian.install .sisx application/vnd.symbian.install .x_t application/x-x_t .ipa application/vnd.iphone .apk application/vnd.android.package-archive .xap application/x-silverlight-app