Hi, I was just wondering if there was a way to make custom structures spawn in a specific vanilla biome.

Yes

 

BiomeGenBase biomegenbase = world.getWorldChunkManager().getBiomeGenAt(i, j);
if(biomeGenBase instanceof BiomeGenForest)
{
   //generate structure
}

Thanks, that seemed to work. But is there a way to make it spawn in like a hillside as well? I tried this:

	protected int[] GetValidSpawnBlocks() {
	return new int[] { Block.dirt.blockID };
}

public boolean LocationIsValidSpawn(World world, int x, int y, int z) {
	int checkID = world.getBlockId(x, y, z + 2);
	int checkID2 = world.getBlockId(x, y + 5, z + 1);

	if (checkID != Block.grass.blockID || (checkID2 != Block.dirt.blockID && checkID2 != Block.stone.blockID)) {
		return false;
	} else {
		return true;
	}
}

public WorldGenDungeon() {
}

public boolean generate(World world, Random rand, int i, int j, int k) {
	// check that each corner is one of the valid spawn blocks
	if (!LocationIsValidSpawn(world, i, j, k + 2) || !LocationIsValidSpawn(world, i, j + 5, k + 1)) {
		return false;
	}

And then the generation underneath that. This is in a custom class called WorldGenDungeon.