"　　项目的缓存中用到了 rocksdb，实例化时报错了： [链接] Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'helloContext' def ...."

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加载rocksdb实例报错：java.lang.UnsatisfiedLinkError: C:\Users\Administrator\AppData\Local\Temp\librocksdbjni3696928169151614297.dll

　　项目的缓存中用到了 rocksdb，实例化时报错了：

Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'helloContext' defined in class path resource [spring-core.xml]: Invocation of init method failed; nested exception is java.lang.UnsatisfiedLinkError: C:\Users\Administrator\AppData\Local\Temp\librocksdbjni3696928169151614297.dll: �
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1628)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:555)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:351)
... 48 more
Caused by: java.lang.UnsatisfiedLinkError: C:\Users\Administrator\AppData\Local\Temp\librocksdbjni3696928169151614297.dll: �
at java.lang.ClassLoader$NativeLibrary.load(Native Method)
at java.lang.ClassLoader.loadLibrary0(ClassLoader.java:1941)
at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1824)
at java.lang.Runtime.load0(Runtime.java:809)
at java.lang.System.load(System.java:1086)
at org.rocksdb.NativeLibraryLoader.loadLibraryFromJar(NativeLibraryLoader.java:78)
at org.rocksdb.NativeLibraryLoader.loadLibrary(NativeLibraryLoader.java:56)
at org.rocksdb.RocksDB.loadLibrary(RocksDB.java:64)
at org.rocksdb.RocksDB.<clinit>(RocksDB.java:35)
at org.rocksdb.Options.<clinit>(Options.java:25)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeCustomInitMethod(AbstractAutowireCapableBeanFactory.java:1758)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1695)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1624)
... 55 more

　　问题现象：类加载失败引起的一系列后续问题：类找不到、实例加载失败、容器加载失败。

　　问题定位：代码跟踪：

public class Options extends RocksObject
    implements DBOptionsInterface<Options>, ColumnFamilyOptionsInterface<Options>,
    MutableColumnFamilyOptionsInterface<Options> {
  static {
    RocksDB.loadLibrary();
  }

　　进入到 RocksDB 类，实例化 rocksDB，先读取是否已加载标志位，没加载过才会去加载:

/**
   * Loads the necessary library files.
   * Calling this method twice will have no effect.
   * By default the method extracts the shared library for loading at
   * java.io.tmpdir, however, you can override this temporary location by
   * setting the environment variable ROCKSDB_SHAREDLIB_DIR.
   */
  public static void loadLibrary() {if (libraryLoaded.get() == LibraryState.LOADED) {return;}
if (libraryLoaded.compareAndSet(LibraryState.NOT_LOADED,
    LibraryState.LOADING)) {
  final String tmpDir = System.getenv("<span style="background-color: rgba(255, 255, 0, 1)">ROCKSDB_SHAREDLIB_DIR</span>");
  // loading possibly necessary libraries.
  for (final CompressionType compressionType : CompressionType.values()) {
    try {
      if (compressionType.getLibraryName() != null) {
        System.loadLibrary(compressionType.getLibraryName());
      }
    } catch (UnsatisfiedLinkError e) {
      // since it may be optional, we ignore its loading failure here.
    }
  }
  try {
    NativeLibraryLoader.getInstance().loadLibrary(tmpDir);
  } catch (IOException e) {
    libraryLoaded.set(LibraryState.NOT_LOADED);
    throw new RuntimeException("Unable to load the RocksDB shared library"
        + e);
  }

  libraryLoaded.set(LibraryState.LOADED);
  return;
}

while (libraryLoaded.get() == LibraryState.LOADING) {
  try {
    Thread.sleep(10);
  } catch(final InterruptedException e) {
    //ignore
  }
}

}

　　注意一点，临时目录是从环境变量 ROCKSDB_SHAREDLIB_DIR 读取的，所以可以通过设置该环境变量改变临时目录路径（日志里的 C:\Users\Administrator\AppData\Local\Temp\librocksdbjni3696928169151614297.dll）。接着进入到 NativeLibraryLoader 看 RocksDB 的实例化详情：

/**
   * Firstly attempts to load the library from <i>java.library.path</i>,
   * if that fails then it falls back to extracting
   * the library from the classpath
   * {@link org.rocksdb.NativeLibraryLoader#loadLibraryFromJar(java.lang.String)}
   *
   * @param tmpDir A temporary directory to use
   *   to copy the native library to when loading from the classpath.
   *   If null, or the empty string, we rely on Java's
   *   {@link java.io.File#createTempFile(String, String)}
   *   function to provide a temporary location.
   *   The temporary file will be registered for deletion
   *   on exit.
   *
   * @throws java.io.IOException if a filesystem operation fails.
   */
  public synchronized void loadLibrary(final String tmpDir) throws IOException {
    try {
        System.loadLibrary(sharedLibraryName);
    } catch(final UnsatisfiedLinkError ule1) {
      try {System.loadLibrary(jniLibraryName);
      } catch(final UnsatisfiedLinkError ule2) {loadLibraryFromJar(tmpDir);
      }
    }
  }
/**

Attempts to extract the native RocksDB library
from the classpath and load it

@param tmpDir A temporary directory to use
to copy the native library to. If null,
or the empty string, we rely on Java's
{@link java.io.File#createTempFile(String, String)}
function to provide a temporary location.
The temporary file will be registered for deletion
on exit.

@throws java.io.IOException if a filesystem operation fails.
*/
void loadLibraryFromJar(final String tmpDir)
throws IOException {
if (!initialized) {
System.load(loadLibraryFromJarToTemp(tmpDir).getAbsolutePath());
initialized = true;
}
}

File loadLibraryFromJarToTemp(final String tmpDir)
throws IOException {
final File temp;
if (tmpDir == null || tmpDir.isEmpty()) {
temp = File.createTempFile(tempFilePrefix, tempFileSuffix);
} else {
temp = new File(tmpDir, jniLibraryFileName);
if (temp.exists() && !temp.delete()) {
throw new RuntimeException("File:" + temp.getAbsolutePath()
+ "already exists and cannot be removed.");
}
if (!temp.createNewFile()) {
throw new RuntimeException("File:" + temp.getAbsolutePath()
+ "could not be created.");
}
}
if (!temp.exists()) {
  throw new RuntimeException("File " + temp.getAbsolutePath() + " does not exist.");
} else {
  temp.deleteOnExit();
}

// attempt to copy the library from the Jar file to the temp destination
try (final InputStream is = getClass().getClassLoader().
  getResourceAsStream(jniLibraryFileName)) {
  if (is == null) {
    throw new RuntimeException(jniLibraryFileName + " was not found inside JAR.");
  } else {
    Files.copy(is, temp.toPath(), StandardCopyOption.REPLACE_EXISTING);
  }
}

return temp;

}

　　先从 sharedLibraryName（rocksdbjni）实例化，再从 jniLibraryName（rocksdbjni-win64）实例化，最后从临时目录实例化。第一步是从 JDK 环境变量（即 java.library.path，本机是 D:\Dev\Java\jdk1.8.0_102\jre\bin）加载 rocksdbjni，失败再从类路径加载 rocksdbjni-win64，最后从本地临时目录加载 librocksdbjni-win64.dll。这三部曲是 rocksDB 实例化的精髓。而最后一步是最复杂的，需要创建一个临时文件，然后从 rocksdbjni 这个 jar 包中把 librocksdbjni-win64.dll 复制到本地 C:\Users\Administrator\AppData\Local\Temp\librocksdbjni3696928169151614297.dll，最后用 org.rocksdb.NativeLibraryLoader 这个类加载器来加载 librocksdbjni3696928169151614297.dll 得到 rocksdb 的实例，然后把这个 DLL 文件加入到加载类库中，后续容器重复加载时无需再次复制 DLL 文件。

　　上面的三部曲都需要利用 java.lang 包的工具来进行类加载，用的到类有 System、Runtime 和 ClassLoader：

    /**
     * Loads the native library specified by the filename argument.  The filename
     * argument must be an absolute path name.
     *
     * If the filename argument, when stripped of any platform-specific library
     * prefix, path, and file extension, indicates a library whose name is,
     * for example, L, and a native library called L is statically linked
     * with the VM, then the JNI_OnLoad_L function exported by the library
     * is invoked rather than attempting to load a dynamic library.
     * A filename matching the argument does not have to exist in the
     * file system.
     * See the JNI Specification for more details.
     *
     * Otherwise, the filename argument is mapped to a native library image in
     * an implementation-dependent manner.
     *
     * <p>
     * The call <code>System.load(name)</code> is effectively equivalent
     * to the call:
     * <blockquote><pre>
     * Runtime.getRuntime().load(name)
     * </pre></blockquote>
     *
     * @param      filename   the file to load.
     * @exception  SecurityException  if a security manager exists and its
     *             <code>checkLink</code> method doesn't allow
     *             loading of the specified dynamic library
     * @exception  UnsatisfiedLinkError  if either the filename is not an
     *             absolute path name, the native library is not statically
     *             linked with the VM, or the library cannot be mapped to
     *             a native library image by the host system.
     * @exception  NullPointerException if <code>filename</code> is
     *             <code>null</code>
     * @see        java.lang.Runtime#load(java.lang.String)
     * @see        java.lang.SecurityManager#checkLink(java.lang.String)
     */
    @CallerSensitive
    public static void load(String filename) {Runtime.getRuntime().load0(Reflection.getCallerClass(), filename);}

    /**
     * Loads the native library specified by the filename argument.  The filename
     * argument must be an absolute path name.
     * (for example
     * <code>Runtime.getRuntime().load("/home/avh/lib/libX11.so");</code>).
     *
     * If the filename argument, when stripped of any platform-specific library
     * prefix, path, and file extension, indicates a library whose name is,
     * for example, L, and a native library called L is statically linked
     * with the VM, then the JNI_OnLoad_L function exported by the library
     * is invoked rather than attempting to load a dynamic library.
     * A filename matching the argument does not have to exist in the file
     * system. See the JNI Specification for more details.
     *
     * Otherwise, the filename argument is mapped to a native library image in
     * an implementation-dependent manner.
     * <p>
     * First, if there is a security manager, its <code>checkLink</code>
     * method is called with the <code>filename</code> as its argument.
     * This may result in a security exception.
     * <p>
     * This is similar to the method {@link #loadLibrary(String)}, but it
     * accepts a general file name as an argument rather than just a library
     * name, allowing any file of native code to be loaded.
     * <p>
     * The method {@link System#load(String)} is the conventional and
     * convenient means of invoking this method.
     *
     * @param      filename   the file to load.
     * @exception  SecurityException  if a security manager exists and its
     *             <code>checkLink</code> method doesn't allow
     *             loading of the specified dynamic library
     * @exception  UnsatisfiedLinkError  if either the filename is not an
     *             absolute path name, the native library is not statically
     *             linked with the VM, or the library cannot be mapped to
     *             a native library image by the host system.
     * @exception  NullPointerException if <code>filename</code> is
     *             <code>null</code>
     * @see        java.lang.Runtime#getRuntime()
     * @see        java.lang.SecurityException
     * @see        java.lang.SecurityManager#checkLink(java.lang.String)
     */
    @CallerSensitive
    public void load(String filename) {load0(Reflection.getCallerClass(), filename);}
synchronized void load0(Class&lt;?&gt; fromClass, String filename) {
    SecurityManager security = System.getSecurityManager();
    if (security != null) {
        security.checkLink(filename);
    }
    if (!(new File(filename).isAbsolute())) {
        throw new UnsatisfiedLinkError(
            "Expecting an absolute path of the library: " + filename);
    }
    ClassLoader.loadLibrary(fromClass, filename, true);
}</pre>

    // 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);
        }
        if (loader != null) {String libfilename = loader.findLibrary(name);
            if (libfilename != null) {File libfile = new File(libfilename);
                if (!libfile.isAbsolute()) {
                    throw new UnsatisfiedLinkError("ClassLoader.findLibrary failed to return an absolute path:" + libfilename);
                }
                if (loadLibrary0(fromClass, libfile)) {return;}
                throw new UnsatisfiedLinkError("Can't load " + libfilename);
            }
        }
        for (int i = 0 ; i < sys_paths.length ; i++) {File libfile = new File(sys_paths[i], System.mapLibraryName(name));
            if (loadLibrary0(fromClass, libfile)) {return;}
            libfile = ClassLoaderHelper.mapAlternativeName(libfile);
            if (libfile != null && loadLibrary0(fromClass, libfile)) {return;}
        }
        if (loader != null) {for (int i = 0 ; i < usr_paths.length ; i++) {File libfile = new File(usr_paths[i],
                                        System.mapLibraryName(name));
                if (loadLibrary0(fromClass, libfile)) {return;}
                libfile = ClassLoaderHelper.mapAlternativeName(libfile);
                if (libfile != null && loadLibrary0(fromClass, libfile)) {return;}
            }
        }
        // Oops, it failed
        throw new UnsatisfiedLinkError("no" + name + "in java.library.path");
    }

    private static boolean loadLibrary0(Class<?> fromClass, final File file) {
        // Check to see if we're attempting to access a static library
        String name = findBuiltinLib(file.getName());
        boolean isBuiltin = (name != null);
        if (!isBuiltin) {
            boolean exists = AccessController.doPrivileged(new PrivilegedAction<Object>() {public Object run() {return file.exists() ? Boolean.TRUE : null;
                    }})
                != null;
            if (!exists) {return false;}
            try {name = file.getCanonicalPath();
            } catch (IOException e) {return false;}
        }
        ClassLoader loader =
            (fromClass == null) ? null : fromClass.getClassLoader();
        Vector<NativeLibrary> libs =
            loader != null ? loader.nativeLibraries : systemNativeLibraries;
        synchronized (libs) {int size = libs.size();
            for (int i = 0; i < size; i++) {NativeLibrary lib = libs.elementAt(i);
                if (name.equals(lib.name)) {return true;}
            }
        synchronized (loadedLibraryNames) {
            if (loadedLibraryNames.contains(name)) {
                throw new UnsatisfiedLinkError
                    ("Native Library " +
                     name +
                     " already loaded in another classloader");
            }
            /* If the library is being loaded (must be by the same thread,
             * because Runtime.load and Runtime.loadLibrary are
             * synchronous). The reason is can occur is that the JNI_OnLoad
             * function can cause another loadLibrary invocation.
             *
             * Thus we can use a static stack to hold the list of libraries
             * we are loading.
             *
             * If there is a pending load operation for the library, we
             * immediately return success; otherwise, we raise
             * UnsatisfiedLinkError.
             */
            int n = nativeLibraryContext.size();
            for (int i = 0; i &lt; n; i++) {
                NativeLibrary lib = nativeLibraryContext.elementAt(i);
                if (name.equals(lib.name)) {
                    if (loader == lib.fromClass.getClassLoader()) {
                        return true;
                    } else {
                        throw new UnsatisfiedLinkError
                            ("Native Library " +
                             name +
                             " is being loaded in another classloader");
                    }
                }
            }
            NativeLibrary lib = new NativeLibrary(fromClass, name, isBuiltin);
            nativeLibraryContext.push(lib);
            try {
                lib.load(name, isBuiltin);
            } finally {
                nativeLibraryContext.pop();
            }
            if (lib.loaded) {
                loadedLibraryNames.addElement(name);
                libs.addElement(lib);
                return true;
            }
            return false;
        }
    }
}</pre>

　　这里拿三部曲最后一部举例说明下：参数 fromClass 就是调用 System.load 方法所在的类，这里是 rocksDB 的加载库类 org.rocksdb.NativeLibraryLoader，另一个参数 File 就是 C:\Users\Administrator\AppData\Local\Temp\librocksdbjni3696928169151614297.dll，从 NativeLibraryLoader 类加载 librocksdbjni5581206381055061043.dll 后，把这个 DLL 文件放入 loadedLibraryNames 这个 Vector，后续重复加载直接从该 Vector 取，无需再从 rocksdbjni 的 jar 包中复制 DLL 文件。

　　跟踪到这里，问题代码已经浮出水面：

lib.load(name, isBuiltin);

　　这个 load 方法是 ClassLoader 类的内部类 NativeLibrary 的原生方法，动态链接库加载就是从这里进入 DLL 文件里具体加载方法的

native void load(String name, boolean isBuiltin);

　　从这个 load 出来就是 UnsatisfiedLinkError。所以问题的焦点转移到了 DLL 文件本身，用 Dependency Walker 打开一看，有错误提示：

Error: Modules with different CPU types were found.

　　继续跟进，发现其中一个文件叫 IEShims.dll 显示是 X86，而我本机是 X64 的。替换该文件，Dependency Walker 不再报错，但跑代码发现依然报错。

　　问题解决：捕获 rockdbs 异常，记录日志，跳过，继续启动加载 spring 容器。该 DLL 的加载涉及太多因素，无法一一排查，只能吃掉异常来规避，rocksdb 加载失败对主业务无太大影响。

加载rocksdb实例报错：java.lang.UnsatisfiedLinkError: C:\Users\Administrator\AppData\Local\Temp\librocksdbjni3696928169151614297.dll

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加载rocksdb实例报错：java.lang.UnsatisfiedLinkError: C:\Users\Administrator\AppData\Local\Temp\librocksdbjni3696928169151614297.dll