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这里从源码学习一下rocksdb是如何加载本地类库的。

maven

<!-- https://mvnrepository.com/artifact/org.rocksdb/rocksdbjni -->
        <dependency>
            <groupId>org.rocksdb</groupId>
            <artifactId>rocksdbjni</artifactId>
            <version>5.5.1</version>
        </dependency>

加载

private enum LibraryState {
    NOT_LOADED,
    LOADING,
    LOADED
  }
private static AtomicReference<LibraryState> libraryLoaded
      = new AtomicReference<>(LibraryState.NOT_LOADED);

  static {
    RocksDB.loadLibrary();
  }

  /**
   * 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("ROCKSDB_SHAREDLIB_DIR");
      // 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
      }
    }
  }

这里设计了一个AtomicReference<LibraryState>，用来避免重复加载

NativeLibraryLoader

/**
 * This class is used to load the RocksDB shared library from within the jar.
 * The shared library is extracted to a temp folder and loaded from there.
 */
public class NativeLibraryLoader {
  //singleton
  private static final NativeLibraryLoader instance = new NativeLibraryLoader();
  private static boolean initialized = false;

  private static final String sharedLibraryName = Environment.getSharedLibraryName("rocksdb");
  private static final String jniLibraryName = Environment.getJniLibraryName("rocksdb");
  private static final String jniLibraryFileName = Environment.getJniLibraryFileName("rocksdb");
  private static final String tempFilePrefix = "librocksdbjni";
  private static final String tempFileSuffix = Environment.getJniLibraryExtension();

  /**
   * Get a reference to the NativeLibraryLoader
   *
   * @return The NativeLibraryLoader
   */
  public static NativeLibraryLoader getInstance() {
    return instance;
  }

  /**
   * 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;
  }

  /**
   * Private constructor to disallow instantiation
   */
  private NativeLibraryLoader() {
  }
}

这个采用了单例的模式，另外加载的代码结构比较清晰

/**
   * 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);
      }
    }
  }

首先从java.library.path加载，加载失败的话，从classpath的jar包加载

Environment设计

public static String getJniLibraryName(final String name) {
    if (isUnix()) {
      final String arch = is64Bit() ? "64" : "32";
      if(isPowerPC()) {
        return String.format("%sjni-linux-%s", name, ARCH);
      } else {
        return String.format("%sjni-linux%s", name, arch);
      }
    } else if (isMac()) {
      return String.format("%sjni-osx", name);
    } else if (isAix() && is64Bit()) {
      return String.format("%sjni-aix64", name);
    } else if (isSolaris()) {
      final String arch = is64Bit() ? "64" : "32";
      return String.format("%sjni-solaris%s", name, arch);
    } else if (isWindows() && is64Bit()) {
      return String.format("%sjni-win64", name);
    }

    throw new UnsupportedOperationException(String.format("Cannot determine JNI library name for ARCH='%s' OS='%s' name='%s'", ARCH, OS, name));
  }

单独的一个Environment来构造跨平台的类库路径