了解Mac上的Oracle Java [英] Understanding Oracle's Java on Mac

问题描述

我已经在OS X上使用Java很多年了，最近苹果公司在默认情况下停止使用Java时我会让操作系统为我安装（当然是苹果公司的产品）。

I've been using Java on OS X for many, many years and recently when Apple stopped including Java by default I let the OS go and install it for me (Apple's variety, of course).

所以现在我正在使用OS X 10.8，我需要安装Java 7，所以我只是以DMG形式获得了Oracle的Update 15并运行了安装程序。它更新了我的/ usr / bin / java（和相关文件），指向这里：

So now I'm using OS X 10.8 and I need to install Java 7 so I just got Oracle's Update 15 in DMG form and ran the installer. It updated my /usr/bin/java (and related files) to point here:

/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/java

追溯到'/ System / Library / Frameworks /JavaVM.framework/Versions的所有内容都指向'Current'或'CurrentJDK'，前者是'A'的链接（这是Oracle的Java 7，我可以告诉它，不知道为什么它是'A'）而后者是'/System/Library/Java/JavaVirtualMachines/1.6.0.jdk'中Apple的Java 6的链接。

Tracing this back to '/System/Library/Frameworks/JavaVM.framework/Versions' everything either points to 'Current' or 'CurrentJDK', the former being a link to 'A' (which is Oracle's Java 7, from what I can tell, not sure why it is 'A') and the latter being a link to Apple's Java 6 in '/System/Library/Java/JavaVirtualMachines/1.6.0.jdk'.

现在这真的很令人困惑但是这甚至不是我的问题。看来这里安装了一个Java 7：

Now this is all really confusing but this isn't even my question yet. It appears there is a Java 7 installed here:

/System/Library/Frameworks/JavaVM.framework/Versions/A

但是这里还安装了Java 7：

But there is also a Java 7 installed here:

/Library/Java/JavaVirtualMachines/jdk1.7.0_15.jdk

在两者中查找'java'并打印出版本会产生相同的版本和版本（java版本1.7.0_15），但是，当对文件进行散列时它们是不同的。

Finding 'java' in both and printing out the version yields the same version and build (java version "1.7.0_15"), however, when hashing the files they are different.

那么这是否意味着Oracle在两个不同的地方安装了Java 7？如果是这样，为什么？我应该使用哪个？为什么有些东西仍然指向Java 6（CurrentJDK）。

So does this mean Oracle installed Java 7 in two different places? If so, why? Which am I supposed to use? And why do some things still point to Java 6 (CurrentJDK).

我查看了甲骨文的网站，但没有任何内容可以解决。

I've looked on Oracle's website but nothing there clears anything up.

推荐答案

Oracle的JVM只安装在一个位置。你被误导了！

Oracle's JVM is only installed in one location. You've been misled!

如你所知， / usr / bin 中的Java命令是符号链接到 /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands 中的二进制文件。该目录中的二进制文件是存根应用程序，它们确定要使用哪个Java VM *，然后在该VM版本中执行相应的实际二进制文件。这就是为什么 /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands 中的所有二进制文件的大小几乎相同，尽管事实上你是这样的期望他们实现完全不同的功能。

As you've noted, the Java commands in /usr/bin are symlinks to binaries in /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands. The binaries within that directory are stub applications that determine which Java VM to use*, and then exec the corresponding real binary within that VM version. This is why all of the binaries within /System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands are almost identical in size, despite the fact that you'd expect them to be implementing quite different functionality.

你可以通过 dtrace 来看到这一点：

You can see this in action by using dtrace:

mrowe@angara:~$ sudo dtrace -n 'syscall::posix_spawn:entry { trace(copyinstr(arg1)); }' -c "/usr/bin/java -version"
dtrace: description 'syscall::posix_spawn:entry ' matched 1 probe
dtrace: pid 44727 has exited
CPU     ID                    FUNCTION:NAME
  8    619                posix_spawn:entry   /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/bin/java

给定的 dtrace 调用打印出 posix_spawn 的路径参数由 java -version 调用。在我的例子中，存根应用程序在 /System/Library/Java/JavaVirtualMachines/1.6.0.jdk 中找到了Apple的Java 1.6运行时，并且正在调用该版本的 java 命令。

The given dtrace invocation prints out the path argument to posix_spawn when it is called by java -version. In my case the stub application has found Apple's Java 1.6 runtime in /System/Library/Java/JavaVirtualMachines/1.6.0.jdk and is invoking that version of the java command.

存根二进制文件还有另一个好处：当他们检测到没有安装Java VM时，它们会提示用户安装一个。

The stub binaries also have another benefit: when they detect that no Java VM is installed they will prompt the user to install one.

至于 CurrentJDK 符号链接，尽管我可以告诉它为了倒退 - 当Apple是OS X上JVM的唯一来源时，与过去的兼容性。

As for the CurrentJDK symlink, as best as I can tell this for sake of backwards-compatibility with the past when Apple was the only source of the JVM on OS X.

*考虑因素的组合确定应该使用哪个Java VM时。如果设置了 JAVA_HOME （尝试 JAVA_HOME = / tmp java ）。如果未设置 JAVA_HOME ，则会发现系统上所有虚拟机的列表。如果设置了 JAVA_VERSION 和 JAVA_ARCH 环境变量，则将虚拟机列表过滤为特定版本和支持的架构。然后根据体系结构（更喜欢64位超过32位）和版本（更新更好）对结果列表进行排序，并返回最佳匹配。

* A combination of factors are considered when determining which Java VM should be used. JAVA_HOME is used if set (try JAVA_HOME=/tmp java). If JAVA_HOME is not set then the list of all virtual machines on the system is discovered. The JAVA_VERSION and JAVA_ARCH environment variables are used, if set, to filter the list of virtual machines to a particular version and supported architecture. The resulting list is then sorted by architecture (preferring 64-bit over 32-bit) and version (newer is better), and the best match is returned.

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