当我还年幼的时候，我很任性，复制数组也是，写一个for循环，来回倒腾，后来长大了，就发现了System.arraycopy的好处。
为了测试俩者的区别我写了一个简单赋值int[100000]的程序来对比，并且中间使用了nanoTime来计算时间差：
程序如下：
int[] a = new int[100000]; for(int i=0;i<a.length;i++){ a[i] = i; } int[] b = new int[100000]; int[] c = new int[100000]; for(int i=0;i<c.length;i++){ c[i] = i; } int[] d = new int[100000]; for(int k=0;k<10;k++){ long start1 = System.nanoTime(); for(int i=0;i<a.length;i++){ b[i] = a[i]; } long end1 = System.nanoTime(); System.out.PRintln("end1 - start1 = "+(end1-start1)); long start2 = System.nanoTime(); System.arraycopy(c, 0, d, 0, 100000); long end2 = System.nanoTime(); System.out.println("end2 - start2 = "+(end2-start2)); System.out.println(); }
为了避免内存不稳定干扰和运行的偶然性结果，我在一开始的时候把所有空间申明完成，并且只之后循环10次执行，得到如下结果：
end1 - start1 = 366806 end2 - start2 = 109154 end1 - start1 = 380529 end2 - start2 = 79849 end1 - start1 = 421422 end2 - start2 = 68769 end1 - start1 = 344463 end2 - start2 = 72020 end1 - start1 = 333174 end2 - start2 = 77277 end1 - start1 = 377335 end2 - start2 = 82285 end1 - start1 = 370608 end2 - start2 = 66937 end1 - start1 = 349067 end2 - start2 = 86532 end1 - start1 = 389974 end2 - start2 = 83362 end1 - start1 = 347937 end2 - start2 = 63638
可以看出，System.arraycopy的性能很不错，为了看看究竟这个底层是如何处理的，我找到openJDK的一些代码留恋了一些：
System.arraycopy是一个native函数，需要看native层的代码：
public static native void arraycopy(Object src, int srcPos, Object dest, int destPos, int length);
找到对应的openjdk6-src/hotspot/src/share/vm/prims/jvm.cpp，这里有JVM_ArrayCopy的入口：
JVM_ENTRY(void, JVM_ArrayCopy(JNIEnv *env, jclass ignored, jobject src, jint src_pos, jobject dst, jint dst_pos, jint length)) JVMWrapper("JVM_ArrayCopy"); // Check if we have null pointers if (src == NULL || dst == NULL) { THROW(vmSymbols::java_lang_NullPointerException()); } arrayOop s = arrayOop(JNIHandles::resolve_non_null(src)); arrayOop d = arrayOop(JNIHandles::resolve_non_null(dst)); assert(s->is_oop(), "JVM_ArrayCopy: src not an oop"); assert(d->is_oop(), "JVM_ArrayCopy: dst not an oop"); // Do copy Klass::cast(s->klass())->copy_array(s, src_pos, d, dst_pos, length, thread); JVM_END
前面的语句都是判断，知道最后的copy_array(s, src_pos, d, dst_pos, length, thread)是真正的copy，进一步看这里，在openjdk6-src/hotspot/src/share/vm/oops/typeArrayKlass.cpp中：
void typeArrayKlass::copy_array(arrayOop s, int src_pos, arrayOop d, int dst_pos, int length, TRAPS) { assert(s->is_typeArray(), "must be type array"); // Check destination if (!d->is_typeArray() || element_type() != typeArrayKlass::cast(d->klass())->element_type()) { THROW(vmSymbols::java_lang_ArrayStoreException()); } // Check is all offsets and lengths are non negative if (src_pos < 0 || dst_pos < 0 || length < 0) { THROW(vmSymbols::java_lang_ArrayIndexOutOfBoundsException()); } // Check if the ranges are valid if ( (((unsigned int) length + (unsigned int) src_pos) > (unsigned int) s->length()) || (((unsigned int) length + (unsigned int) dst_pos) > (unsigned int) d->length()) ) { THROW(vmSymbols::java_lang_ArrayIndexOutOfBoundsException()); } // Check zero copy if (length == 0) return; // This is an attempt to make the copy_array fast. int l2es = log2_element_size(); int ihs = array_header_in_bytes() / WordSize; char* src = (char*) ((oop*)s + ihs) + ((size_t)src_pos << l2es); char* dst = (char*) ((oop*)d + ihs) + ((size_t)dst_pos << l2es); Copy::conjoint_memory_atomic(src, dst, (size_t)length << l2es);//还是在这里处理copy }
这个函数之前的仍然是一堆判断，直到最后一句才是真实的拷贝语句。
在openjdk6-src/hotspot/src/share/vm/utilities/copy.cpp中找到对应的函数：
// Copy bytes; larger units are filled atomically if everything is aligned. void Copy::conjoint_memory_atomic(void* from, void* to, size_t size) { address src = (address) from; address dst = (address) to; uintptr_t bits = (uintptr_t) src | (uintptr_t) dst | (uintptr_t) size; // (Note: We could improve performance by ignoring the low bits of size, // and putting a short cleanup loop after each bulk copy loop. // There are plenty of other ways to make this faster also, // and it's a slippery slope. For now, let's keep this code simple // since the simplicity helps clarify the atomicity semantics of // this Operation. There are also CPU-specific assembly versions // which may or may not want to include such optimizations.) if (bits % sizeof(jlong) == 0) { Copy::conjoint_jlongs_atomic((jlong*) src, (jlong*) dst, size / sizeof(jlong)); } else if (bits % sizeof(jint) == 0) { Copy::conjoint_jints_atomic((jint*) src, (jint*) dst, size / sizeof(jint)); } else if (bits % sizeof(jshort) == 0) { Copy::conjoint_jshorts_atomic((jshort*) src, (jshort*) dst, size / sizeof(jshort)); } else { // Not aligned, so no need to be atomic. Copy::conjoint_jbytes((void*) src, (void*) dst, size); } }
上面的代码展示了选择哪个copy函数，我们选择conjoint_jints_atomic，在openjdk6-src/hotspot/src/share/vm/utilities/copy.hpp进一步查看：
// jints, conjoint, atomic on each jint static void conjoint_jints_atomic(jint* from, jint* to, size_t count) { assert_params_ok(from, to, LogBytesPerInt); pd_conjoint_jints_atomic(from, to, count); }
继续向下查看，在openjdk6-src/hotspot/src/cpu/zero/vm/copy_zero.hpp中：
static void pd_conjoint_jints_atomic(jint* from, jint* to, size_t count) { _Copy_conjoint_jints_atomic(from, to, count); }
继续向下查看，在openjdk6-src/hotspot/src/os_cpu/linux_zero/vm/os_linux_zero.cpp中：
void _Copy_conjoint_jints_atomic(jint* from, jint* to, size_t count) { if (from > to) { jint *end = from + count; while (from < end) *(to++) = *(from++); } else if (from < to) { jint *end = from; from += count - 1; to += count - 1; while (from >= end) *(to--) = *(from--); } }
可以看到，直接就是内存块赋值的逻辑了，这样避免很多引用来回倒腾的时间，必然就变快了。
