我的代碼是：

if (frameRGBABuffer == null) {

frameRGBABuffer = ByteBuffer.allocateDirect(cameraHeight * cameraWidth * 4)

.order(ByteOrder.nativeOrder());

}

Log.d("tag",frameRGBABuffer.array().length)

我的相機(jī)分辨率是1280×720,因此frameRGBABuffer應(yīng)該分配3686400字節(jié)的空間.

但是很奇怪frameRGBABuffer.array()的長(zhǎng)度是3686407.為什么它有額外的7個(gè)字節(jié)空格？

順便說(shuō)一下,frameRGBABuffer.array()不會(huì)拋出異常并返回帶有數(shù)據(jù)的byte []

似乎Android分配了7個(gè)額外的空間來(lái)處理對(duì)齊.

源代碼是：

MemoryRef(int capacity) {

VMRuntime runtime = VMRuntime.getRuntime();

buffer = (byte[]) runtime.newNonMovableArray(byte.class, capacity + 7);

allocatedAddress = runtime.addressOf(buffer);

// Offset is set to handle the alignment: http://b/16449607

offset = (int) (((allocatedAddress + 7) & ~(long) 7) - allocatedAddress);

isAccessible = true;

isFreed = false;

}

解決方法:

這是它背后的代碼(JVM,不是Android,但在Android上可能類似)：

DirectByteBuffer(int cap) { // package-private

super(-1, 0, cap, cap);

boolean pa = VM.isDirectMemoryPageAligned();

int ps = Bits.pageSize();

long size = Math.max(1L, (long)cap + (pa ? ps : 0));

Bits.reserveMemory(size, cap);

long base = 0;

try {

base = unsafe.allocateMemory(size);

} catch (OutOfMemoryError x) {

Bits.unreserveMemory(size, cap);

throw x;

}

unsafe.setMemory(base, size, (byte) 0);

if (pa && (base % ps != 0)) {

// Round up to page boundary

address = base + ps - (base & (ps - 1));

} else {

address = base;

}

cleaner = Cleaner.create(this, new Deallocator(base, size, cap));

att = null;

VM.isDirectMemoryPageAligned()< ---是關(guān)鍵

// User-controllable flag that determines if direct buffers should be page

// aligned. The "-XX:+PageAlignDirectMemory" option can be used to force

// buffers, allocated by ByteBuffer.allocateDirect, to be page aligned.

這是性能低級(jí)的東西.

標(biāo)簽：bytebuffer,android,java

來(lái)源： https://codeday.me/bug/20190828/1747249.html

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