java - 使用 java.library.path 和 LD_LIBRARY_PATH 的区别

Question

设置JVM参数有区别吗

-Djava.library.path=/path 

在 JVM 启动并设置 Linux 环境变量

export LD_LIBRARY_PATH=/path 

在 JVM 启动之前？

这两种方法的优点/缺点是什么？

Answer 1

第一种形式

-Djava.library.path=/path

会在java字节码级别处理，System.loadLibrary会调用Runtime.loadLibary，然后会调用java/lang/ClassLoader.loadLibrary。在函数调用 ClassLoader.loadLibrary 中，将检查系统属性 java.library.path 以获取库的完整路径并将此完整路径传递给 native 代码调用系统 api dlopen/dlsym，最终使库加载。可以从OpenJDK浏览源码存储库。以下代码片段是我从链接中复制的片段。

这种形式的优点是，如果您的库路径存在一些问题，您将在 Java 代码中收到错误或警告或异常。

// Invoked in the java.lang.Runtime class to implement load and loadLibrary.
static void loadLibrary(Class fromClass, String name,
                        boolean isAbsolute) {
    ClassLoader loader =
        (fromClass == null) ? null : fromClass.getClassLoader();
    if (sys_paths == null) {
        usr_paths = initializePath("java.library.path");
        sys_paths = initializePath("sun.boot.library.path");
    }
    if (isAbsolute) {
        if (loadLibrary0(fromClass, new File(name))) {
            return;
        }
        throw new UnsatisfiedLinkError("Can't load library: " + name);
    }
// ....

第二种形式

export LD_LIBRARY_PATH=/path

会在native中处理，根据dlopen/dlsym

的文档

 dlopen()
   The function dlopen() loads the dynamic library file named by the null-terminated string filename and returns an opaque  "handle"  for  the
   dynamic  library.   If  filename is NULL, then the returned handle is for the main program.  If filename contains a slash ("/"), then it is
   interpreted as a (relative or absolute) pathname.  Otherwise, the dynamic linker searches for the library as follows (see ld.so(8) for fur‐
   ther details):

   o   (ELF  only)  If  the  executable  file for the calling program contains a DT_RPATH tag, and does not contain a DT_RUNPATH tag, then the
       directories listed in the DT_RPATH tag are searched.

   o   If, at the time that the program was started, the environment variable LD_LIBRARY_PATH was defined to contain a colon-separated list of
       directories, then these are searched.  (As a security measure this variable is ignored for set-user-ID and set-group-ID programs.)

以这种方式，如果您的库路径存在一些问题并且系统无法加载您的库，系统将不会提供太多线索并会默默地失败(我猜)。这取决于是否实现LD_LIBRARY_PATH，Android 没有使用LD_LIBRARY_PATH 来确定库位置，您可以从here 看到Android 的实现。 .