来自：程序员乔戈里

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什么是boolean类型，根据官方文档的描述^[1]：

boolean: The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isn't something that's precisely defined.

谷歌翻译一下：

布尔类型：布尔数据类型只有两个可能的值：真和假。使用此数据类型为跟踪真/假条件的简单标记。这种数据类型就表示这一点信息，但是它的“大小”并不是精确定义

stackoverflow就有关于boolean占几个字节的讨论。 what-is-the-size-of-a-boolean-variable-in-java^[2] 其中有一个高赞回答：

 /***出自公众号：程序员乔戈里*/
class LotsOfBooleans
{boolean a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af;boolean b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf;boolean c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf;boolean d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df;boolean e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef;
}class LotsOfInts
{int a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, aa, ab, ac, ad, ae, af;int b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf;int c0, c1, c2, c3, c4, c5, c6, c7, c8, c9, ca, cb, cc, cd, ce, cf;int d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, da, db, dc, dd, de, df;int e0, e1, e2, e3, e4, e5, e6, e7, e8, e9, ea, eb, ec, ed, ee, ef;
}public class Test
{private static final int SIZE = 100000;public static void main(String[] args) throws Exception{LotsOfBooleans[] first = new LotsOfBooleans[SIZE];LotsOfInts[] second = new LotsOfInts[SIZE];System.gc();long startMem = getMemory();for (int i=0; i < SIZE; i++){first[i] = new LotsOfBooleans();}System.gc();long endMem = getMemory();System.out.println ("Size for LotsOfBooleans: " + (endMem-startMem));System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE)));System.gc();startMem = getMemory();for (int i=0; i < SIZE; i++){second[i] = new LotsOfInts();}System.gc();endMem = getMemory();System.out.println ("Size for LotsOfInts: " + (endMem-startMem));System.out.println ("Average size: " + ((endMem-startMem) / ((double)SIZE)));// Make sure nothing gets collectedlong total = 0;for (int i=0; i < SIZE; i++){total += (first[i].a0 ? 1 : 0) + second[i].a0;}System.out.println(total);}private static long getMemory(){Runtime runtime = Runtime.getRuntime();return runtime.totalMemory() - runtime.freeMemory();}
}

运行结果

Size for LotsOfBooleans: 8257544
Average size: 82.57544
Size for LotsOfInts: 33599984
Average size: 335.99984

Java虚拟机规范一书提到 :

•在Java虚拟机中没有任何供 boolean值专用的字节码指令,Java语言表达式所操作的 boolean值,在编译之后都使用Java虚拟机中的int数据类型来代替。•Java虚拟机直接支持 boolean类型的数组,虚拟机的 navarra指令参见第6章的newarray小节可以创建这种数组。boolean类型数组的访问与修改共用byte类型数组的baload和 bastore指令。

•因为在虚拟机规范中说了，boolean值在编译之后都使用Java虚拟机中的int数据类型来代替，而int是4个字节，那么boolean值就是4个字节。•boolean类型数组的访问与修改共用byte类型数组的baload和 bastore指令，因为两者共用，只有两者字节一样才能通用呀，所以byte数组中一个byte是1个字节，那么boolean数组中boolean是1个字节。•总结：boolean在数组情况下为1个字节，单个boolean为4个字节。

Java规范中，没有明确指出boolean的大小。在《Java虚拟机规范》给出了单个boolean占4个字节，和boolean数组1个字节的定义，具体 还要看虚拟机实现是否按照规范来，所以1个字节、4个字节都是有可能的^[3]

References

[1] 官方文档的描述: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
[2] what-is-the-size-of-a-boolean-variable-in-java: https://stackoverflow.com/questions/383551/what-is-the-size-of-a-boolean-variable-in-java
[3] 所以1个字节、4个字节都是有可能的: https://blog.csdn.net/makingadream/article/details/53100237

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