问题
I have a requirement to generate unique Long ids for my database primary key column.
I thought i can use UUID.randomUUID().getMostSignificantBits() but sometimes its generating some negative long also which is problem for me.
Is it possible to generate only positive long from UUID ?There will be like billions of entries so i want that each generated key must be unique.
回答1:
Take a look at http://commons.apache.org/sandbox/commons-id//index.html It has a LongGenerator that can give you exactly what you need.
In addition if you are using Hibernate then you can ask it to generate IDs for you (it has several algorithms you can choose from), in if not you can just take a look at their implementation for example http://grepcode.com/file/repo1.maven.org/maven2/hibernate/hibernate/2.1.8/net/sf/hibernate/id/TableHiLoGenerator.java#TableHiLoGenerator)
回答2:
UUID.randomUUID().getMostSignificantBits() & Long.MAX_VALUE
The reason why this works is, when you do bitwise & with 1 it allows the same digit to pass as it is and when you do bitwise & with 0 it blocks it and result is 0. Now, Long.MAX_Value in binary is
0111111111111111111111111111111111111111111111111111111111111111
this is 0 followed by 63 1s (total is 64 bits, it's long in java)
So when you bitwise & a number X with this above number then you will get the same number X except that the leftmost bit is now turned into a zero. Which means you've only changed the sign of that number and not the value.
回答3:
As the others have written, long does not have enough space for a unique number. But in many cases a number may be unique enough for a specific use. For example, a timestamp with the nanosecond precision is often good enough. To get it, shift the current milliseconds 20 bits left to allocate space for nanoseconds and then overlay it with the nanoseconds:
(System.currentTimeMillis() << 20) | (System.nanoTime() & ~9223372036854251520L);
The nano & ~9223372036854251520L part takes the current nanoseconds and sets the first 44 bytes to 0, leaving only the right 20 bits which represent nanoseconds up to one millisecond (999999 nanos) It is the same as:
nanoseconds & ~1111111111111111111111111111111111111111111100000000000000000000
Side note: nanoseconds should not be used to represent the current time because their starting point is not fixed in time and because they are recycled when they reach the maximum.
You can use any other bit manipulation. It is usually good to take into account the current time and something else such as the current thread id, process id, ip.
回答4:
I just came across this solution. I am for the time being trying to understand the solution.It says Java implementation of twitter snowflake. 64 bit sequential ID generator based on twitter snowflake ID generation algorithm.
https://github.com/Predictor/javasnowflake
Any suggestions are welcome.
来源：https://stackoverflow.com/questions/15184820/how-to-generate-unique-positive-long-using-uuid