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advantages/disadvantages是什么？

Answer 1

第一种形式

-Djava.library.path=/path

将在 java 字节码级别处理，System.loadLibrary 将调用 Runtime.loadLibary，然后将调用 java/lang/ClassLoader.loadLibrary。在函数调用ClassLoader.loadLibrary中，会检查系统属性java.library.path获取库的完整路径，并将此完整路径传递给本机代码调用系统apidlopen/dlsym，最终使库加载。您可以从 OpenJDK 存储库浏览源代码。以下代码片段是我从 link.

复制的片段

这种形式的优点是，如果您的库路径有问题，您会在 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

根据dlopen/dlsym

的文档，以native方式处理

 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）。看有没有实现LD_LIBRARY_PATH，Android没有用LD_LIBRARY_PATH判断库位置，可以从here看Android的实现.

Answer 2

Java 可以明确加载 -Djava.library.path=... 列出的库正如 alijandro 所述。

比如mq系列使用bindings模式，可以指定需要的库路径-Djava.library.path=/opt/mq/java/lib，mqseries加载库。

如果库不是从 java 明确加载的，即必须使用依赖库，则必须使用 LD_LIBRARY_PATH 才能在 jvm 中使用该库。

使用 java.library.path 和 LD_LIBRARY_PATH 的区别

Difference between using java.library.path and LD_LIBRARY_PATH

java

linux

java-native-interface

shared-libraries