0xCAFEBABE。
]]>181783497276652981这么一个Magic Number, 没有定义为常量, 也没有对这个数做什么特别的说明, 感觉很奇怪, 于是好奇心驱使我扒一扒这个数字的由来：// java version: 1.8.0_251
public class Random implements java.io.Serializable {
// ...
/**
* The internal state associated with this pseudorandom number generator.
* (The specs for the methods in this class describe the ongoing
* computation of this value.)
*/
private final AtomicLong seed;
/**
* Creates a new random number generator. This constructor sets
* the seed of the random number generator to a value very likely
* to be distinct from any other invocation of this constructor.
*/
public Random() {
this(seedUniquifier() ^ System.nanoTime());
}
private static long seedUniquifier() {
// L'Ecuyer, "Tables of Linear Congruential Generators of
// Different Sizes and Good Lattice Structure", 1999
for (;;) {
long current = seedUniquifier.get();
long next = current * 181783497276652981L;
if (seedUniquifier.compareAndSet(current, next))
return next;
}
}
private static final AtomicLong seedUniquifier
= new AtomicLong(8682522807148012L);
// ...
}0xCAFEBABE。
]]>java.util.Random中的nextInt(int bound)方法, 有这么两行代码让我虎躯一震。int m = bound - 1;
if ((bound & m) == 0) // i.e., bound is a power of 2这就能检测一个数是不是2的幂了？？？
]]>
]]><dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-mongodb</artifactId>
</dependency>总是报错：
com.mongodb.MongoSecurityException: Exception authenticating MongoCredential{mechanism=null, userName='test', source='test', password=<hidden>, mechanismProperties={}}
at com.mongodb.connection.SaslAuthenticator.wrapException(SaslAuthenticator.java:162)
at com.mongodb.connection.SaslAuthenticator.access$200(SaslAuthenticator.java:39)
at com.mongodb.connection.SaslAuthenticator$1.run(SaslAuthenticator.java:68)
at com.mongodb.connection.SaslAuthenticator$1.run(SaslAuthenticator.java:46)
at com.mongodb.connection.SaslAuthenticator.doAsSubject(SaslAuthenticator.java:168)
at com.mongodb.connection.SaslAuthenticator.authenticate(SaslAuthenticator.java:46)
at com.mongodb.connection.DefaultAuthenticator.authenticate(DefaultAuthenticator.java:32)
at com.mongodb.connection.InternalStreamConnectionInitializer.authenticateAll(InternalStreamConnectionInitializer.java:122)
at com.mongodb.connection.InternalStreamConnectionInitializer.initialize(InternalStreamConnectionInitializer.java:52)
at com.mongodb.connection.InternalStreamConnection.open(InternalStreamConnection.java:127)
at com.mongodb.connection.DefaultServerMonitor$ServerMonitorRunnable.run(DefaultServerMonitor.java:114)
at java.lang.Thread.run(Thread.java:748)
Caused by: com.mongodb.MongoCommandException: Command failed with error 18: 'Authentication failed.' on server localhost:27017. The full response is { "ok" : 0.0, "errmsg" : "Authentication failed.", "code" : 18, "codeName" : "AuthenticationFailed" }
at com.mongodb.connection.ProtocolHelper.getCommandFailureException(ProtocolHelper.java:164)
at com.mongodb.connection.InternalStreamConnection.receiveCommandMessageResponse(InternalStreamConnection.java:295)
at com.mongodb.connection.InternalStreamConnection.sendAndReceive(InternalStreamConnection.java:255)
at com.mongodb.connection.CommandHelper.sendAndReceive(CommandHelper.java:84)
at com.mongodb.connection.CommandHelper.executeCommand(CommandHelper.java:34)
at com.mongodb.connection.SaslAuthenticator.sendSaslContinue(SaslAuthenticator.java:123)
at com.mongodb.connection.SaslAuthenticator.access$100(SaslAuthenticator.java:39)
at com.mongodb.connection.SaslAuthenticator$1.run(SaslAuthenticator.java:65)
... 9 common frames omitted