一直以来都是用jdk1.5,这次重返电信由于其系统是在jdk1.4上编译的,编译的时候出现了unsupported major.minor version49.0的错误,上网查看了一下还是一个很普遍的错误,捣鼓了两天终于捣鼓出一些东西,现分享给大家。
何谓 major.minor,且又居身于何处呢?先感性认识并找到 major.minor 来。顺便写一段 代码,然后用 JDK 1.5 的编译器编译成class,用UltraEdit或者其他能打开非十进制文件的软件打开此class,见下图:
从上图中我们看出来了什么是 major.minor version 了,它相当于一个软件的主次版本号,只是在这里是标识的一个 Java Class 的主版本号和次版本号,同时我们看到 minor_version 为 0x0000,major_version 为 0x0031,转换为十制数分别为0 和 49,即 major.minor 就是 49.0 了。
现在不妨从 JDK 1.1 到 JDK 1.7 编译器编译出的 class 的默认 minor.major version 吧。(又走到 Sun 的网站上翻腾出我从来都没用过的古董来)
JDK 编译器版本 |
target 参数 |
十六进制 minor.major |
十进制 minor.major |
jdk1.1.8 |
不能带 target 参数 |
00 03 00 2D |
45.3 |
jdk1.2.2 |
不带(默认为 -target 1.1) |
00 03 00 2D |
45.3 |
jdk1.2.2 |
-target 1.2 |
00 00 00 2E |
46.0 |
jdk1.3.1_19 |
不带(默认为 -target 1.1) |
00 03 00 2D |
45.3 |
jdk1.3.1_19 |
-target 1.3 |
00 00 00 2F |
47.0 |
j2sdk1.4.2_10 |
不带(默认为 -target 1.2) |
00 00 00 2E |
46.0 |
j2sdk1.4.2_10 |
-target 1.4 |
00 00 00 30 |
48.0 |
jdk1.5.0_11 |
不带(默认为 -target 1.5) |
00 00 00 31 |
49.0 |
jdk1.5.0_11 |
-target 1.4 -source 1.4 |
00 00 00 30 |
48.0 |
jdk1.6.0_01 |
不带(默认为 -target 1.6) |
00 00 00 32 |
50.0 |
jdk1.6.0_01 |
-target 1.5 |
00 00 00 31 |
49.0 |
jdk1.6.0_01 |
-target 1.4 -source 1.4 |
00 00 00 30 |
48.0 |
jdk1.7.0 |
不带(默认为 -target 1.6) |
00 00 00 32 |
50.0 |
jdk1.7.0 |
-target 1.7 |
00 00 00 33 |
51.0 |
jdk1.7.0 |
-target 1.4 -source 1.4 |
00 00 00 30 |
48.0 |
Apache Harmony 5.0M3 |
不带(默认为 -target 1.2) |
00 00 00 2E |
46.0 |
Apache Harmony 5.0M3 |
-target 1.4 |
00 00 00 30 |
48.0 |
当然你也可以用其他方法查看版本号,比如javap -verbose XXXX(class名)。
那么现在如果碰到这种问题该知道如何解决了吧,还会像我所见到有些兄弟那样,去找个 1.4 的 JDK 下载安装,然后用其重新编译所有的代码吗?且不说这种方法的繁琐,而且web应用程序还不一定能成功,其实大可不必如此费神,我们一定还记得 javac 还有个 -target 参数,对啦,可以继续使用 1.5 JDK,编译时带上参数 -target 1.4 -source 1.4 就 OK 啦,不过你一定要对哪些 API 是 1.5 JDK 加入进来的了如指掌,不能你的 class 文件拿到 JVM 1.4 下就会 method not found。目标 JVM 是 1.3 的话,编译选项就用 -target 1.3 -source 1.3 了。
相应的如果使用 ant ,它的 javac 任务也可对应的选择 target 和 source
<javac target="1.4" source="1.4" ............................/>
如果是在开发中,可以肯定的是现在真正算得上是 JAVA IDE 对于工程也都有编译选项设置目标代码的。例如 Eclipse 的项目属性中的 Java Compiler 设置,如图:
其实理解 major.minor 就像是我们可以这么想像,同样是微软件的程序,32 位的应用程序不能拿到 16 位系统中执行那样。
如果我们发布前了解到目标 JVM 版本,知道怎么从 java class 文件中看出 major.minor 版本来,就不用等到服务器报出异常才着手去解决,也就能预知到可能发生的问题。
其他时候遇到这个问题应具体解决,总之问题的根由是低版本的 JVM 无法加载高版本的 class 文件造成的,找到高版本的 class 文件处理一下就行了。
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